Turn on -Wundefined-reinterpret-cast.

It was not a fan of this (blatant) aliasing.
I suspect this best_non_simd_srcover_srgb_srgb() function has several
other aliasing issues that use undefined behavior, but this is all it's
complaining about for now.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2606

Change-Id: I25a8800e810bccf5068c8a10e9c8c8f565e57304
Reviewed-on: https://skia-review.googlesource.com/2606
Commit-Queue: Mike Klein <mtklein@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Herb Derby <herb@google.com>
2 files changed