mac: Set better target flags when compiling on different hosts.

Bug: 1147069
Change-Id: If01314a9a6a7ba359f79cbd58ed9e732cf70928f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538720
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827718}

build/config/mac/BUILD.gn

@@ -15,22 +15,21 @@ config("compiler") {
 
   # CPU architecture.
   if (current_cpu == "x64") {
-    common_mac_flags += [
-      "-arch",
-      "x86_64",
-    ]
+    clang_arch = "x86_64"
   } else if (current_cpu == "x86") {
-    common_mac_flags += [
-      "-arch",
-      "i386",
-    ]
+    clang_arch = "i386"
   } else if (current_cpu == "arm64") {
+    clang_arch = current_cpu
+  } else {
+    assert(false, "unknown current_cpu $current_cpu")
+  }
+  if (host_os == "mac") {
     common_mac_flags += [
       "-arch",
-      current_cpu,
+      clang_arch,
     ]
   } else {
-    assert(false, "unknown current_cpu $current_cpu")
+    common_mac_flags += [ "--target=$clang_arch-apple-macos" ]
   }
 
   # This is here so that all files get recompiled after an Xcode update.