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