bitset: implement BIT_TEST_CLR_ATOMIC & BIT_TEST_SET_ATOMIC

That is, provide wrappers around the atomic_testandclear and
atomic_testandset primitives.

Submitted by: jeff
Reviewed by: cem, kib, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22702