mac: enough to make `target_cpu = "arm64"` make it through `gn gen`

Doesn't build at all, but hey, it's a start.

Bug: 1098899
Change-Id: I63500b3650b315c47219614cc9ae9a3ddbcf42c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264499
Commit-Queue: Mark Mentovai <mark@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782116}

build/config/BUILDCONFIG.gn

@@ -237,7 +237,7 @@ if (target_os == "android") {
   _default_toolchain = "//build/toolchain/mac:ios_clang_$target_cpu"
 } else if (target_os == "mac") {
   assert(host_os == "mac", "Mac cross-compiles are unsupported.")
-  _default_toolchain = host_toolchain
+  _default_toolchain = "//build/toolchain/mac:clang_$target_cpu"
 } else if (target_os == "win") {
   # On Windows, we use the same toolchain for host and target by default.
   # Beware, win cross builds have some caveats, see docs/win_cross.md
@@ -585,10 +585,9 @@ foreach(_target_type,
       # On Android, write shared library output file to metadata. We will use
       # this information to, for instance, collect all shared libraries that
       # should be packaged into an APK.
-      if (!defined(invoker.metadata) && is_android && (_target_type ==
-                                                       "shared_library" ||
-                                                       _target_type ==
-                                                       "loadable_module")) {
+      if (!defined(invoker.metadata) && is_android &&
+          (_target_type == "shared_library" ||
+           _target_type == "loadable_module")) {
         _output_name = _target_name
         if (defined(invoker.output_name)) {
           _output_name = invoker.output_name

build/config/mac/BUILD.gn

@@ -24,6 +24,13 @@ config("compiler") {
       "-arch",
       "i386",
     ]
+  } else if (current_cpu == "arm64") {
+    common_mac_flags += [
+      "-arch",
+      current_cpu,
+    ]
+  } else {
+    assert(false, "unknown current_cpu $current_cpu")
   }
 
   # This is here so that all files get recompiled after an Xcode update.

build/toolchain/mac/BUILD.gn

@@ -477,6 +477,13 @@ mac_toolchain("clang_arm") {
   }
 }
 
+mac_toolchain("clang_arm64") {
+  toolchain_args = {
+    current_cpu = "arm64"
+    current_os = "mac"
+  }
+}
+
 mac_toolchain("clang_x64") {
   toolchain_args = {
     current_cpu = "x64"

third_party/zlib/BUILD.gn

@@ -90,6 +90,8 @@ if (use_arm_neon_optimizations) {
         defines += [ "ARMV8_OS_ANDROID" ]
       } else if (is_linux || is_chromeos) {
         defines += [ "ARMV8_OS_LINUX" ]
+      } else if (is_mac) {
+        defines += [ "ARMV8_OS_MACOS" ]
       } else if (is_fuchsia) {
         defines += [ "ARMV8_OS_FUCHSIA" ]
       } else if (is_win) {