Separate target architecture type and width into separate variables.

Also add support for building mac in 64 bit mode.
Review URL: https://codereview.appspot.com/6343050

git-svn-id: http://skia.googlecode.com/svn/trunk@4385 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gyp/common.gypi b/gyp/common.gypi
index 717e6eb..85c590d 100644
--- a/gyp/common.gypi
+++ b/gyp/common.gypi
@@ -24,6 +24,9 @@
         ['skia_angle and not skia_os == "win"',
           {'error': '<!(skia_angle=1 only supported with skia_os="win".)'
         }],
+        ['skia_arch_width != 32 or skia_arch_width != 64',
+          {'error': '<!(skia_arch_width can only be 32 or 64 bits not <(skia_arch_width) bits)'
+        }],
       ],
     },
     'includes': [