lib/libc/tests/string/strcspn_test.c: add test for correct match order

This new unit test verifies that if there are multiple
matches, the first match is returned, ignoring later
matches.

Approved by: mjg (blanket, via IRC)
MFC after: 1 week
MFC to: stable/14

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/strcspn_test.c: extend tests to catch previous bug

This extends the strcspn() unit tests to catch mistakes in the
implementation that only appear when a mismatch occurs in a certain
position of the string against a certain position of the set.

See also: 52d4a4d4e0dedc72bc33082a3f84c2d0fd6f2cbb
Sponsored by: The FreeBSD Foundation
Approved by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D41821

lib/libc/tests/string: derive strspn(3) tests from strcspn(3) tests

To cover the new optimised amd64 strspn(3) SIMD implementation, extend
the previously written strcspn(3) unit test to also cover strspn(3).

Sponsored by: The FreeBSD Foundation
Approved by: mjg
MFC after: 1 week
MFC to: stable/14
Differential Revision: https://reviews.freebsd.org/D41567

lib/libc/tests/string: add unit tests for strcspn(3)

We currently use the NetBSD test suite to cover strcspn(3). It only
contains a very rudimentary test of this function. This all new set
of unit tests for the FreeBSD test suite should cover many more edge
cases relating to alignment issues.

Sponsored by: The FreeBSD Foundation
Approved by: mjg
MFC after: 1 week
MFC to: stable/14
Differential Revision: https://reviews.freebsd.org/D41557