Fix AVX512 feature detection in the ninja file, slightly

cpuinfo on my machine includes both "flags" and "vmx flags". Previously,
the first grep would emit both, and the "look for lines without avx512f"
would succeed -- even though the actual flags line has that feature.

Now, we only look for lines that actually start with "flags".

Change-Id: I77c772417cc3436e872c62f44e85e28416a8874d
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/771517
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed