Change unittest flags to decimal so they can be used for --libyuv_cpu_info=
Add environment variables to disable AVX 512 bits.

Bug: libyuv:784
Test: LibYUVBaseTest.TestCpuHas
Change-Id: Iea6704368fbe9f6d3395933da7993fb2a3453225
Reviewed-on: https://chromium-review.googlesource.com/957704Reviewed-by: richard winterton <rrwinterton@gmail.com>