[GN] Define SK_BUILD_NO_OPTS when building skia on iOS.

skia/skia_common.gypi

Index: skia/skia_common.gypi
diff --git a/skia/skia_common.gypi b/skia/skia_common.gypi
index 21801cc5a1e052279296d48bfde0f64e07ded596..c57c5952e0ae4d511b14639b599544fe2deb449e 100644
--- a/skia/skia_common.gypi
+++ b/skia/skia_common.gypi
@@ -31,7 +31,12 @@
     }],
     [ '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',
       ],
     }],
     [ 'OS != "mac"', {