gn build: Make builtin library build on macOS

[thakis]

Summary

For now, it only builds the x86_64 slice.

[phosek]

LGTM

llvm/utils/gn/build/toolchain/BUILD.gn

unix_toolchain("unix") {
   }
 
   toolchain_args = {
     current_os = host_os
     current_cpu = host_cpu
   }
 }
 
+# This template defines a toolchain that uses just-built clang and lld
+# as compiler and linker.
 template("stage2_unix_toolchain") {
   unix_toolchain(target_name) {
     forward_variables_from(invoker, "*")
 
     cc = "bin/clang"
     cxx = "bin/clang++"
     ld = cxx
     if (current_os != "mac") {

llvm/utils/gn/secondary/BUILD.gn

 import("//clang/lib/ARCMigrate/enable.gni")
 import("//clang/lib/StaticAnalyzer/Frontend/enable.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 
 group("default") {
   deps = [
     "//clang-tools-extra/clangd/test",
     "//clang-tools-extra/test",
     "//clang/test",
     "//lld/test",
     "//llvm/test",
   ]
+  if (current_os == "linux" || current_os == "mac") {
+    deps += [ "//compiler-rt" ]
+  }
   if (current_os == "linux") {
     deps += [
-      "//compiler-rt",
       "//libcxx",
       "//libcxxabi",
       "//libunwind",
     ]
   }
   if (current_os == "linux" || current_os == "android") {
     deps += [ "//compiler-rt/test/hwasan" ]
   }

llvm/utils/gn/secondary/compiler-rt/BUILD.gn

 group("compiler-rt") {
+  # In the GN build, compiler-rt is always built by just-built clang and lld.
+  # FIXME: For macOS and iOS builds, depend on lib in all needed target arch
+  # toolchains and then lipo them together for the final output.
   deps = [
     "//compiler-rt/lib(//llvm/utils/gn/build/toolchain:stage2_unix)",
   ]
 }

llvm/utils/gn/secondary/compiler-rt/lib/builtins/BUILD.gn

 import("//compiler-rt/target.gni")
 import("//llvm/utils/gn/build/buildflags.gni")
 
 declare_args() {
   # Skip the atomic builtin (should normally be provided by a shared library).
   compiler_rt_exclude_atomic_builtin = true
 
   # Compile builtins for baremetal.
   compiler_rt_baremetal_build = false
 }
 
 static_library("builtins") {
   output_dir = crt_current_out_dir
+  if (current_os == "mac" ) {
+    output_name = "clang_rt.osx"
+  } else {
     output_name = "clang_rt.builtins$crt_current_target_suffix"
+  }
   complete_static_lib = true
   cflags = [
     "-fPIC",
     "-fno-builtin",
   ]
   if (target_os != "android") {
     cflags += [ "-fvisibility=hidden" ]
   }

llvm/utils/gn/secondary/compiler-rt/target.gni

if (clang_enable_per_target_runtime_dir) {
   crt_current_target_suffix = ""
 } else if (current_os == "linux" || current_os == "android") {
   crt_current_out_dir = "$clang_resource_dir/lib/linux"
 
   crt_current_target_suffix = "-$crt_current_target_arch"
   if (current_os == "android") {
     crt_current_target_suffix += "-android"
   }
+} else if (current_os == "mac") {
+  crt_current_out_dir = "$clang_resource_dir/lib/darwin"
 } else {
   assert(false, "unimplemented current_os " + current_os)
 }