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

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