Add a GYP flag to define SKNX_NO_SIMD when set.

Will use this with a new -SKNX_NO_SIMD bot.

BUG=skia:

Review URL: https://codereview.chromium.org/1227163016

gyp/common_conditions.gypi

@@ -628,6 +628,10 @@
       ],
     }],
 
+    [ 'sknx_no_simd', {
+      'defines': [ 'SKNX_NO_SIMD' ],
+    }],
+
   ], # end 'conditions'
   # The Xcode SYMROOT must be at the root. See build/common.gypi in chromium for more details
   'xcode_settings': {

gyp/common_variables.gypi

@@ -208,6 +208,7 @@
     'skia_gpu%': '<(skia_gpu)',
     'skia_win_exceptions%': 0,
     'skia_win_ltcg%': 1,
+    'sknx_no_simd%': 0,
     'skia_osx_deployment_target%': '<(skia_osx_deployment_target)',
     'skia_profile_enabled%': '<(skia_profile_enabled)',
     'skia_shared_lib%': '<(skia_shared_lib)',