RFC: Test optimizations for multiple architectures

[Roland McGrath]

> If I am adding a testsuite for rawmemchr, why do I have to know what
> mask permutations are interesting to run? It is irrelevant to the rawmemchr
> testsuite.

When you are adding an ifunc implementation of rawmemchr or anything else,
you are responsible for knowing what mask permutations are relevant to
making sure all the CPU-specific implementations you have added are covered
by the test suite.