rx: atomic bit ops

Add rx_atomic_test_bit, rx_atomic_set_bit, and rx_atomic_clear_bit
to provide bitwise operations over atomic types. These allow the
use of atomic flag variables.

Uses native platform atomic operations wherever these are available,
otherwise falls back to our 'standard' MUTEX based implementation.

LICENSE BSD

Change-Id: I161631b2408e85f757ff04bf831a631ccb1f930d
Reviewed-on: http://gerrit.openafs.org/8936
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>