Reland "gn build: Fix support for building the builtins for baremetal."

Our support for building for baremetal was conditional on a default
off arg and would have failed to build if you had somehow arranged
to pass the correct --target flag; presumably nobody noticed because
nobody was turning it on. A better approach is to model baremetal
as a separate "OS" called "baremetal" and build it in the same way
as we cross-compile for other targets. That's what this patch does.
I only hooked up the arm64 target but others can be added.

Relanding after fixing Mac build breakage in D123244.

Differential Revision: https://reviews.llvm.org/D122862

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

@@ -245,6 +245,13 @@ if (host_os == "mac") {
   }
 }
 
+stage2_unix_toolchain("stage2_baremetal_aarch64") {
+  toolchain_args = {
+    current_os = "baremetal"
+    current_cpu = "arm64"
+  }
+}
+
 template("win_toolchain") {
   toolchain(target_name) {
     # https://groups.google.com/a/chromium.org/d/msg/gn-dev/F_lv5T-tNDM

llvm/utils/gn/build/toolchain/target_flags.gni

@@ -42,6 +42,8 @@ if (current_os == "android") {
       "x86_64",
     ]
   }
+} else if (current_os == "baremetal") {
+  target_flags += [ "--target=$llvm_current_triple" ]
 }
 
 if (current_cpu == "x86") {

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

@@ -12,6 +12,9 @@ if (current_os == "win") {
   supported_toolchains = [ "//llvm/utils/gn/build/toolchain:stage2_unix" ]
 }
 supported_toolchains += supported_android_toolchains
+if (llvm_build_AArch64) {
+  supported_toolchains += [ "//llvm/utils/gn/build/toolchain:stage2_baremetal_aarch64" ]
+}
 group("compiler-rt") {
   deps = [ "//compiler-rt/include($host_toolchain)" ]
   foreach(toolchain, supported_toolchains) {

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

@@ -2,21 +2,23 @@ group("lib") {
   deps = [
     "//compiler-rt/lib/asan:ignorelist($host_toolchain)",
     "//compiler-rt/lib/cfi:ignorelist($host_toolchain)",
-    "//compiler-rt/lib/profile",
   ]
   if (current_os == "linux") {
     deps += [ "//compiler-rt/lib/msan" ]
   }
   if (current_os == "linux" || current_os == "android") {
     deps += [ "//compiler-rt/lib/ubsan_minimal" ]
   }
-  if (current_os != "win") {
-    deps += [
-      "//compiler-rt/lib/asan",
-      "//compiler-rt/lib/builtins",
-    ]
+  if (current_os != "win" && current_os != "baremetal") {
+    deps += [ "//compiler-rt/lib/asan" ]
     if (current_cpu == "x64" || current_cpu == "arm64") {
       deps += [ "//compiler-rt/lib/tsan/rtl" ]
     }
   }
+  if (current_os != "win") {
+    deps += [ "//compiler-rt/lib/builtins" ]
+  }
+  if (current_os != "baremetal") {
+    deps += [ "//compiler-rt/lib/profile" ]
+  }
 }

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

@@ -4,9 +4,6 @@ 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") {
@@ -170,17 +167,17 @@ static_library("builtins") {
   ]
 
   if (current_os != "fuchsia") {
+    sources += [ "clear_cache.c" ]
+  }
+
+  if (current_os != "fuchsia" && current_os != "baremetal") {
     sources += [
       "emutls.c",
       "enable_execute_stack.c",
       "eprintf.c",
     ]
   }
 
-  if (current_os != "fuchsia" && !compiler_rt_baremetal_build) {
-    sources += [ "clear_cache.c" ]
-  }
-
   if (current_os == "mac" || current_os == "ios") {
     sources += [
       "atomic_flag_clear.c",

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

@@ -28,6 +28,9 @@ if (clang_enable_per_target_runtime_dir) {
   }
 } else if (current_os == "ios" || current_os == "mac") {
   crt_current_out_dir = "$clang_resource_dir/lib/darwin"
+} else if (current_os == "baremetal") {
+  crt_current_out_dir = "$clang_resource_dir/lib/baremetal"
+  crt_current_target_suffix = "-$crt_current_target_arch"
 } else if (current_os == "win") {
   crt_current_out_dir = "$clang_resource_dir/lib/windows"
   crt_current_target_suffix = "-$crt_current_target_arch"

llvm/utils/gn/secondary/llvm/triples.gni

@@ -23,6 +23,8 @@ if (current_cpu == "x86") {
     llvm_current_triple = "aarch64-linux-android29"
   } else if (current_os == "ios" || current_os == "mac") {
     llvm_current_triple = "arm64-apple-darwin"
+  } else if (current_os == "baremetal") {
+    llvm_current_triple = "aarch64-elf"
   } else if (current_os == "linux") {
     llvm_current_triple = "aarch64-unknown-linux-gnu"
   }