[x86] split memcmp tests for 32/64-bit targets; NFC

memcmp is defined as taking a size_t length arg,
so that differs depending on pointer size of the
target.

We casually matched non-compliant function signatures
as memcmp, but that can cause crashing as seen with
PR50850.

If we fix that bug, these tests would no longer be
testing the expected behavior for a 32-bit target,
so I have duplicated all tests and adjusted them
to match the stricter definition of memcmp/bcmp
by changing the length arg to i32 on a 32-bit target.