Fix linker feature detection being performed on wrong linker

Bazel forces use of `lld` and `ld.gold` if found, but performed feature
detection on the default linker instead.

tools/cpp/unix_cc_configure.bzl

@@ -161,10 +161,9 @@ def _is_compiler_option_supported(repository_ctx, cc, option):
     ])
     return result.stderr.find(option) == -1
 
-def _is_linker_option_supported(repository_ctx, cc, option, pattern):
+def _is_linker_option_supported(repository_ctx, cc, force_linker_flags, option, pattern):
     """Checks that `option` is supported by the C linker. Doesn't %-escape the option."""
-    result = repository_ctx.execute([
-        cc,
+    result = repository_ctx.execute([cc] + force_linker_flags + [
         option,
         "-o",
         "/dev/null",
@@ -213,9 +212,9 @@ def _add_compiler_option_if_supported(repository_ctx, cc, option):
     """Returns `[option]` if supported, `[]` otherwise. Doesn't %-escape the option."""
     return [option] if _is_compiler_option_supported(repository_ctx, cc, option) else []
 
-def _add_linker_option_if_supported(repository_ctx, cc, option, pattern):
+def _add_linker_option_if_supported(repository_ctx, cc, force_linker_flags, option, pattern):
     """Returns `[option]` if supported, `[]` otherwise. Doesn't %-escape the option."""
-    return [option] if _is_linker_option_supported(repository_ctx, cc, option, pattern) else []
+    return [option] if _is_linker_option_supported(repository_ctx, cc, force_linker_flags, option, pattern) else []
 
 def _get_no_canonical_prefixes_opt(repository_ctx, cc):
     # If the compiler sometimes rewrites paths in the .d files without symlinks
@@ -420,16 +419,40 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
         False,
     ), ":")
 
+    gold_or_lld_linker_path = (
+        _find_linker_path(repository_ctx, cc, "lld", is_clang) or
+        _find_linker_path(repository_ctx, cc, "gold", is_clang)
+    )
+    cc_path = repository_ctx.path(cc)
+    if not str(cc_path).startswith(str(repository_ctx.path(".")) + "/"):
+        # cc is outside the repository, set -B
+        bin_search_flags = ["-B" + escape_string(str(cc_path.dirname))]
+    else:
+        # cc is inside the repository, don't set -B.
+        bin_search_flags = []
+    if not gold_or_lld_linker_path:
+        ld_path = repository_ctx.path(tool_paths["ld"])
+        if ld_path.dirname != cc_path.dirname:
+            bin_search_flags.append("-B" + str(ld_path.dirname))
+    force_linker_flags = []
+    if gold_or_lld_linker_path:
+        force_linker_flags.append("-fuse-ld=" + gold_or_lld_linker_path)
+
+    # TODO: It's unclear why these flags aren't added on macOS.
+    if bin_search_flags and not darwin:
+        force_linker_flags.extend(bin_search_flags)
     use_libcpp = darwin or bsd
     is_as_needed_supported = _is_linker_option_supported(
         repository_ctx,
         cc,
+        force_linker_flags,
         "-Wl,-no-as-needed",
         "-no-as-needed",
     )
     is_push_state_supported = _is_linker_option_supported(
         repository_ctx,
         cc,
+        force_linker_flags,
         "-Wl,--push-state",
         "--push-state",
     )
@@ -463,21 +486,6 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
         bazel_linklibs,
         False,
     ), ":")
-    gold_or_lld_linker_path = (
-        _find_linker_path(repository_ctx, cc, "lld", is_clang) or
-        _find_linker_path(repository_ctx, cc, "gold", is_clang)
-    )
-    cc_path = repository_ctx.path(cc)
-    if not str(cc_path).startswith(str(repository_ctx.path(".")) + "/"):
-        # cc is outside the repository, set -B
-        bin_search_flags = ["-B" + escape_string(str(cc_path.dirname))]
-    else:
-        # cc is inside the repository, don't set -B.
-        bin_search_flags = []
-    if not gold_or_lld_linker_path:
-        ld_path = repository_ctx.path(tool_paths["ld"])
-        if ld_path.dirname != cc_path.dirname:
-            bin_search_flags.append("-B" + str(ld_path.dirname))
     coverage_compile_flags, coverage_link_flags = _coverage_flags(repository_ctx, darwin)
     print_resource_dir_supported = _is_compiler_option_supported(
         repository_ctx,
@@ -610,19 +618,18 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
             ),
             "%{cxx_flags}": get_starlark_list(cxx_opts + _escaped_cplus_include_paths(repository_ctx)),
             "%{conly_flags}": get_starlark_list(conly_opts),
-            "%{link_flags}": get_starlark_list((
-                ["-fuse-ld=" + gold_or_lld_linker_path] if gold_or_lld_linker_path else []
-            ) + (
+            "%{link_flags}": get_starlark_list(force_linker_flags + (
                 ["-Wl,-no-as-needed"] if is_as_needed_supported else []
             ) + _add_linker_option_if_supported(
                 repository_ctx,
                 cc,
+                force_linker_flags,
                 "-Wl,-z,relro,-z,now",
                 "-z",
             ) + (
                 [
                     "-headerpad_max_install_names",
-                ] if darwin else bin_search_flags + [
+                ] if darwin else [
                     # Gold linker only? Can we enable this by default?
                     # "-Wl,--warn-execstack",
                     # "-Wl,--detect-odr-violations"
@@ -664,6 +671,7 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
                 ["-Wl,-dead_strip"] if darwin else _add_linker_option_if_supported(
                     repository_ctx,
                     cc,
+                    force_linker_flags,
                     "-Wl,--gc-sections",
                     "-gc-sections",
                 ),