Revert to old behavior regarding SK_CPU_ARM and update documentation around SK_CPU_ARM64 on iOS.

R=robertphillips@google.com

Author: djsollen@google.com

Review URL: https://codereview.chromium.org/216823008

git-svn-id: http://skia.googlecode.com/svn/trunk@14057 2bbb7eff-a529-9590-31e7-b0007b416f81

include/core/SkPreConfig.h

@@ -149,7 +149,7 @@
 //////////////////////////////////////////////////////////////////////
 // ARM defines
 
-#if defined(__arm__) && !defined(__APPLE__)
+#if defined(__arm__) && (!defined(__APPLE__) || !TARGET_IPHONE_SIMULATOR)
     #define SK_CPU_ARM
 
     #if defined(__GNUC__)
@@ -180,7 +180,8 @@
     #endif
 #endif
 
-#if defined(__aarch64__) && !defined(__APPLE__)
+// Disable ARM64 optimizations for iOS due to complications regarding gyp and iOS.
+#if defined(__aarch64__) && !defined(SK_BUILD_FOR_IOS)
     #define SK_CPU_ARM64
 #endif