Use atomic_interrupt_fence() instead of bare __compiler_membar()

for the which which definitely use membar to sync with interrupt handlers.

libc and rtld uses of __compiler_membar() seems to want compiler barriers
proper.

The barrier in sched_unpin_lite() after td_pinned decrement seems to be not
needed and removed, instead of convertion.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28956