[GN] Add support for disabling opts via SK_BUILD_NO_OPTS define.

gyp/common_conditions.gypi

Index: gyp/common_conditions.gypi
diff --git a/gyp/common_conditions.gypi b/gyp/common_conditions.gypi
index 21f4c92398a10e35ef8aa5ee0bdd9875ac39e73c..4f387e988380c165e62e08706474f4e08c29688d 100644
--- a/gyp/common_conditions.gypi
+++ b/gyp/common_conditions.gypi
@@ -562,7 +562,12 @@
     [ 'skia_os == "ios"',
       {
         'defines': [
+          # When targetting iOS and using gyp to generate the build files, it is
+          # not possible to select files to build depending on the architecture
+          # (i.e. it is not possible to use hand optimized assembly version). In
+          # that configuration, disable all optimisation.
           'SK_BUILD_FOR_IOS',
+          'SK_BUILD_NO_OPTS',
         ],
         'conditions' : [
           [ 'skia_warnings_as_errors', {