update build for Apple Silicon

You can use this to cross complie from an x86_64 mac if you have
at least XCode12 beta 2 installed, and you set target_cpu = "arm64"
in your gn args.

Change-Id: I3fcdcd162155ac0242c15260994de09177ff2f97
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328659
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>

gn/BUILD.gn

@@ -254,6 +254,23 @@ config("default") {
     #     /System/Library/Frameworks/foo.framework/foo are out of sync.
     #     Falling back to library file for linking.
     ldflags += [ "-Wl,-w" ]
+
+    # As of 11/2020, gn is an x86 binary and defaults the host_cpu to x86_64.
+    # This allows you to build arm64 mac binaries by setting target_cpu = "arm64"
+    if (current_cpu == "arm64") {
+      asmflags += [
+        "-target",
+        "arm64-apple-macos11",
+      ]
+      cflags += [
+        "-target",
+        "arm64-apple-macos11",
+      ]
+      ldflags += [
+        "-target",
+        "arm64-apple-macos11",
+      ]
+    }
   }
 
   if (sanitize != "" && sanitize != "MSVC") {

third_party/libjpeg-turbo/BUILD.gn

@@ -95,7 +95,7 @@ if (skia_use_system_libjpeg_turbo) {
         "../externals/libjpeg-turbo/simd/arm/common/jidctint-neon.c",
         "../externals/libjpeg-turbo/simd/arm/common/jidctred-neon.c",
       ]
-    } else if (current_cpu == "arm64" && !is_ios && !is_win) {
+    } else if (current_cpu == "arm64" && !is_mac && !is_ios && !is_win) {
       sources += [
         "../externals/libjpeg-turbo/simd/arm/arm64/jsimd.c",
         "../externals/libjpeg-turbo/simd/arm/arm64/jsimd_neon.S",

third_party/zlib/BUILD.gn

@@ -40,7 +40,7 @@ if (skia_use_system_zlib) {
         defines += [ "ARMV8_OS_FUCHSIA" ]
       } else if (is_win) {
         defines += [ "ARMV8_OS_WINDOWS" ]
-      } else if (is_ios) {
+      } else if (is_ios || is_mac) {
         # iOS@ARM is a special case where we always have NEON but don't check
         # for crypto extensions.
         defines += [ "ARM_OS_IOS" ]