lib/libc/string/bcmp.c: fix integer overflow bug

bcmp() returned the number of remaining bytes when the main loop exits.
In case of a match, this is zero, else a positive integer. On systems
where SIZE_MAX > INT_MAX, the implicit conversion from size_t to int in
the return value may cause the number of remaining bytes to overflow,
becoming zero and falsely indicating a successful comparison.

Fix the bug by always returning 0 on equality, 1 otherwise.

PR: 272474
Approved by: emaste
Reviewed by: imp
MFC After: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41011