Mov Linux & Mac GN compiler flags to separate configs.

build/config/mac/BUILD.gn

Index: build/config/mac/BUILD.gn
diff --git a/build/config/mac/BUILD.gn b/build/config/mac/BUILD.gn
index 16dc52eb0dda89c2d4aed03f54b4684f53d585e2..c2b7fe9cf480410baead30c182bbf358a730e787 100644
--- a/build/config/mac/BUILD.gn
+++ b/build/config/mac/BUILD.gn
@@ -5,6 +5,48 @@
 import("//build/config/sysroot.gni")
 import("//build/config/mac/mac_sdk.gni")
 
+# This is included by reference in the //build/config/compiler config that
+# is applied to all targets. It is here to separate out the logic.
+#
+# This is applied to BOTH desktop Mac and iOS targets.
+config("compiler") {
+  # These flags are shared between the C compiler and linker.
+  common_mac_flags = []
+
+  # CPU architecture.
+  if (current_cpu == "x64") {
+    common_mac_flags += [
+      "-arch",
+      "x86_64",
+    ]
+  } else if (current_cpu == "x86") {
+    common_mac_flags += [
+      "-arch",
+      "i386",
+    ]
+  } else if (current_cpu == "arm") {
+    # TODO(GYP): we may need to distinguish between "arm64", "armv7",
+    # and "armv7s" for iOS, and hence need multiple current_cpu values
+    # rather than just "arm".
+    common_mac_flags += [
+      "-arch",
+      "arm64",
+    ]
+  }
+
+  asmflags = common_mac_flags
+  cflags = common_mac_flags
+
+  # Without this, the constructors and destructors of a C++ object inside
+  # an Objective C struct won't be called, which is very bad.
+  cflags_objcc = [ "-fobjc-call-cxx-cdtors" ]
+
+  cflags_c = [ "-std=c99" ]
+  cflags_objc = cflags_c
+
+  ldflags = common_mac_flags
+}
+
 # This is included by reference in the //build/config/compiler:runtime_library
 # config that is applied to all targets. It is here to separate out the logic
 # that is Mac-only. Please see that target for advice on what should go in