pw_toolchain: Support replacing GNU libs for ARM

For projects that wish to completely move away from GNU libraries such
as newlib and prefer prebuilt LLVM libraries shipped with toolchain,
provide alternative configs that do not use the python script
`clang_arm_toolchain.py`.

Bug: b/322360978
Change-Id: I83bd1d7830b5749c3fb0dafe25e0e09dc527cbaa
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/250572
Commit-Queue: Prabhu Karthikeyan Rajasekaran <[email protected]>
Lint: Lint 🤖 <[email protected]>
Docs-Not-Needed: Prabhu Karthikeyan Rajasekaran <[email protected]>
Reviewed-by: Armando Montanez <[email protected]>

pw_toolchain/arm_clang/BUILD.gn

@@ -16,6 +16,10 @@ import("//build_overrides/pigweed.gni")
 
 import("clang_config.gni")
 
+declare_args() {
+  pw_toolchain_USE_LLVM_PREBUILT_LIBRARIES = false
+}
+
 cortex_m_common_flags = [
   "-mabi=aapcs",
   "-mthumb",
@@ -49,12 +53,58 @@ config("enable_float_printf") {
   ldflags = [ "-Wl,-u_printf_float" ]
 }
 
-pw_clang_arm_config("cortex_m0plus") {
-  cflags = [ "-mcpu=cortex-m0plus" ]
-  cflags += cortex_m_common_flags
-  cflags += cortex_m_software_fpu_flags
-  asmflags = cflags
-  ldflags = cflags
+if (pw_toolchain_USE_LLVM_PREBUILT_LIBRARIES) {
+  cortex_m_common_link_flags = [
+    "-nostartfiles",
+    "-nostdlib++",
+  ]
+  cortex_m_common_defines = [
+    # This macro is used by libc++ headers and normally would expand to std::_libcpp_verbose_abort,
+    # but we don't define that since we don't build with libc++ at the moment. For now, we can
+    # have this lower to a trap instruction.
+    "_LIBCPP_VERBOSE_ABORT(...)=__builtin_trap()",
+  ]
+  config("cortex_m0plus") {
+    cflags = [
+      "-mcpu=cortex-m0plus",
+      "--target=armv6m-none-eabi",
+    ]
+    cflags += cortex_m_common_flags
+    cflags += cortex_m_software_fpu_flags
+    asmflags = cflags
+    ldflags = cflags
+    ldflags += cortex_m_common_link_flags
+    defines = cortex_m_common_defines
+  }
+
+  config("cortex_m4") {
+    cflags = [
+      "-mcpu=cortex-m4",
+      "--target=armv7m-none-eabi",
+    ]
+    cflags += cortex_m_common_flags
+    cflags += cortex_m_software_fpu_flags
+    asmflags = cflags
+    ldflags = cflags
+    ldflags += cortex_m_common_link_flags
+    defines = cortex_m_common_defines
+  }
+} else {
+  pw_clang_arm_config("cortex_m0plus") {
+    cflags = [ "-mcpu=cortex-m0plus" ]
+    cflags += cortex_m_common_flags
+    cflags += cortex_m_software_fpu_flags
+    asmflags = cflags
+    ldflags = cflags
+  }
+
+  pw_clang_arm_config("cortex_m4") {
+    cflags = [ "-mcpu=cortex-m4" ]
+    cflags += cortex_m_common_flags
+    cflags += cortex_m_software_fpu_flags
+    asmflags = cflags
+    ldflags = cflags
+  }
 }
 
 pw_clang_arm_config("cortex_m3") {
@@ -66,14 +116,6 @@ pw_clang_arm_config("cortex_m3") {
   rustflags = [ "--target=thumbv7m-none-eabi" ]
 }
 
-pw_clang_arm_config("cortex_m4") {
-  cflags = [ "-mcpu=cortex-m4" ]
-  cflags += cortex_m_common_flags
-  cflags += cortex_m_software_fpu_flags
-  asmflags = cflags
-  ldflags = cflags
-}
-
 pw_clang_arm_config("cortex_m4f") {
   cflags = [ "-mcpu=cortex-m4" ]
   cflags += cortex_m_common_flags