mac/arm64: Compile with -mcpu=apple-a12

The baseline CPU for macOS on arm64 is the Apple A12 (Vortex/Tempest),
and clang should default to this when building for this OS/CPU
combination. Apple clang embedded in Xcode 12 does, but trunk clang does
not, instead targeting the Apple A7 (Cyclone), the proper baseline CPU
for iOS on arm64. https://reviews.llvm.org/D82699 would fix it, but it
hasn't gained any traction. Specify the baseline CPU explicitly here.

Bug: 1107587, 1119249
Change-Id: I2a132b00ef9b5c8262aa24f27a7519d83a3df41a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363772
Auto-Submit: Mark Mentovai <mark@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799582}

build/config/mac/BUILD.gn

@@ -28,6 +28,17 @@ config("compiler") {
     common_mac_flags += [
       "-arch",
       current_cpu,
+
+      # The baseline CPU for macOS on arm64 is the Apple A12 (Vortex/Tempest),
+      # and clang should default to this when building for this OS/CPU
+      # combination. Apple clang embedded in Xcode 12 does, but trunk clang does
+      # not, instead targeting the Apple A7 (Cyclone), the proper baseline CPU
+      # for iOS on arm64. https://reviews.llvm.org/D82699 would fix it, but it
+      # hasn't gained any traction. Specify the baseline CPU explicitly here.
+      #
+      # TODO(https://crbug.com/1119249): Remove this when trunk clang has a
+      # better default.
+      "-mcpu=apple-a12",
     ]
   } else {
     assert(false, "unknown current_cpu $current_cpu")