atomic(9): Add relaxed load/store variants from FreeBSD

Obtained from FreeBSD. However, the original atomic_store_ptr() macro
was causing '-Wcast-qual' warnings, so I changed it based on the
NetBSD

atomic(9): Add relaxed load/store variants from FreeBSD

Obtained from FreeBSD. However, the original atomic_store_ptr() macro
was causing '-Wcast-qual' warnings, so I changed it based on the
NetBSD's atomic_store_relaxed().

Actually, I found NetBSD's version more clean, but we import the FreeBSD
version to help import code/drivers in the future.

Discussed-with: dillon

show more ...