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