libc: Have memcmp test what the standard requires

libc's C memcmp currently returns the difference in byte values rather
than just -1/0/1 as the AArch64 assembly implementation, many non-
FreeBSD im

libc: Have memcmp test what the standard requires

libc's C memcmp currently returns the difference in byte values rather
than just -1/0/1 as the AArch64 assembly implementation, many non-
FreeBSD implementations, and compiler built-in optimizations do.

It is a bug for a user to expect memcmp to return the difference in the
byte values as the compiler is free to inline memcmp() with an
implementation that does not do this. Change the test to validate only
what the standard requires.

PR: 289084
Reviewed by: markj, fuz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52502

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