Emit correct darwin_x86_64 CPU value if not using Xcode

Previously, if not using the Xcode toolchain, for example because the
locator failed, the standard Unix C++ toolchain was created with a CPU
value of either `darwin_arm64` or `darwin`, depending on the host
architecture. The latter, when appended to
`@local_config_cc//:cc-compiler-` in the toolchain BUILD template,
results in a reference to a non-existent toolchain.

tools/cpp/lib_cc_configure.bzl

@@ -183,7 +183,7 @@ def get_cpu_value(repository_ctx):
     arch = repository_ctx.os.arch
     if os_name.startswith("mac os"):
         # Check if we are on x86_64 or arm64 and return the corresponding cpu value.
-        return "darwin" + ("_arm64" if arch == "aarch64" else "")
+        return "darwin" + ("_arm64" if arch == "aarch64" else "_x86_64")
     if os_name.find("freebsd") != -1:
         return "freebsd"
     if os_name.find("openbsd") != -1: