Fix optional neon support

On Android, we detect neon support at runtime and switch implementations
based on that. This requires that some parts of skia are compiled with
-mfpu=neon.

To support this, the -mfpu flag is moved out of the 'compiler' config
and into its own config. A target can then remove this config and supply
its own -mfpu flag (without having to duplicate all the other stuff in
the 'compiler' config).

BUG=359249

Review URL: https://codereview.chromium.org/371853007

git-svn-id: http://src.chromium.org/svn/trunk/src/build@286038 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

config/BUILDCONFIG.gn

@@ -310,6 +310,7 @@ _native_compiler_configs = [
   "//build/config:feature_flags",
 
   "//build/config/compiler:compiler",
+  "//build/config/compiler:compiler_arm_fpu",
   "//build/config/compiler:chromium_code",
   "//build/config/compiler:default_include_dirs",
   "//build/config/compiler:default_warnings",

config/arm.gni

@@ -21,7 +21,7 @@ if (cpu_arch == "arm") {
 
   if (is_android) {
     arm_use_neon = false
-    arm_optionally_use_neon = false
+    arm_optionally_use_neon = true
   } else {
     arm_use_neon = true
     arm_optionally_use_neon = true

config/compiler/BUILD.gn

@@ -134,7 +134,6 @@ config("compiler") {
       if (!is_android_webview_build) {
         cflags += [
           "-march=$arm_arch",
-          "-mfpu=$arm_fpu",
           "-mfloat-abi=$arm_float_abi",
         ]
         if (arm_tune != "") {
@@ -314,6 +313,14 @@ config("compiler") {
   }
 }
 
+config("compiler_arm_fpu") {
+  if (cpu_arch == "arm" && !is_android_webview_build) {
+    cflags = [
+      "-mfpu=$arm_fpu",
+    ]
+  }
+}
+
 # runtime_library -------------------------------------------------------------
 #
 # Sets the runtime library and associated options.