Only use _opts unpremul on ARM

The _opts unpremul slows down on x86, but shows an improvement
on ARM. Only allow the _opts code to run on ARM.

Bug: b/325378308
Bug: b/323034773
Change-Id: I6532746b94270a7c89e9e2b48f2f64113cb873ee
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/815938
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed