libc: Purge unneeded cdefs.h

These sys/cdefs.h are not needed. Purge them. They are mostly left-over
from the $FreeBSD$ removal. A few in libc are still required for macros
that cdefs.h defines. Keep those.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D42385

lib/libc/tests/string: expand memcmp test to bcmp, timingsafe_{b,mem}cmp

The four functions more or less perform the same operation.
Reuse the same unit test with slight changes so we can cover
them all. Constant-time operation is not verified for the
timingsafe_* functions.

Sponsored by: The FreeBSD Foundation
Approved by: ngie
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41528

lib/libc/tests/string/memcmp_test.c: extend test to support custom memcmp function

Extend the tests to permit loading an external memcmp function
and testing it over using the libc version. This was added by the
example of other tests in the test suite doing the same thing and
helped tremendously in development.

This change was originally part of D41442 but was taken out to
permit separate review as extrapolated from @ngie's request in
D41349.

Sponsored by: FreeBSD Foundation
Approved by: ngie
Differential Revision: https://reviews.freebsd.org/D41528

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

libc: Add some tests for memcmp().