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: add unit tests for ffs, ffsl, ffsll, fls, flsl, and flsll

Also supply CFLAGS+=-fno-builtin to ensure our unit tests
actually test libc functions and not clang's builtins.

Sponsored by: FreeBSD Foundation
Approved by: kevans
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D40729