gn build: unbreak mac (and maybe win) after r351258, r351277

[thakis]

Summary

The check-hwasan build files assert that current_os == "linux" || current_os == "android", so pull it in only there.

ar is unused on mac, so don't set it in the stage2 toolchain. (It'd be nicer to use llvm-libtool on mac instead of host libtool, but llvm-libtool doesn't seem to understand the -no_warning_for_no_symbols flag.)

[pcc]

LGTM

It ought to be possible to cross-compile the Android runtimes on Mac/Win but that can probably be looked into later.

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

 
 template("stage2_unix_toolchain") {
   unix_toolchain(target_name) {
     forward_variables_from(invoker, "*")
 
     cc = "bin/clang"
     cxx = "bin/clang++"
     ld = cxx
+    if (current_os != "mac") {
       ar = "bin/llvm-ar"
+    }
 
     deps = [
       "//:clang($host_toolchain)",
       "//:lld($host_toolchain)",
-      "//:llvm-ar($host_toolchain)",
     ]
+    if (current_os != "mac") {
+      deps += [ "//:llvm-ar($host_toolchain)" ]
+    }
   }
 }
 
 stage2_unix_toolchain("stage2_unix") {
   toolchain_args = {
     current_os = host_os
     current_cpu = host_cpu
     use_lld = host_os != "mac"

llvm/trunk/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/test",
-    "//compiler-rt/test/hwasan",
     "//lld/test",
     "//llvm/test",
   ]
+  if (current_os == "linux" || current_os == "android") {
+    deps += [ "//compiler-rt/test/hwasan" ]
+  }
 
   testonly = true
 }
 
 # Symlink handling.
 # On POSIX, symlinks to the target can be created before the target exist,
 # and the target can depend on the symlink targets, so that building the
 # target ensures the symlinks exist.