rx: Make rx_atomic.h a shared header

rx: Make rx_atomic.h a shared header

Copy rx_atomic.h into the shared header directory so that it can be
used by other modules within the AFS build. Note that we can't actually
install this header onto the system (and it cannot be included by
headers which are installed) because it uses autoconf defines to control
its behaviour.

Change-Id: If543f998faa0608b4f7efab90db31f072f9e9981
Reviewed-on: http://gerrit.openafs.org/3579
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>