asm-generic/atomic.h: add atomic_set_mask() helper

Since arches are expected to implement this guy, add a common version for
people the same way as atomic_clear_mask is handled.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Arun Sharma <asharma@fb.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>