Fix spinlock implementation for some !solaris sparc platforms.

 Fix spinlock implementation for some !solaris sparc platforms.
 
 Some Sparc CPUs can be run in various coherence models, ranging from
 RMO (relaxed) over PSO (partial) to TSO (total). Solaris has always
 run CPUs in TSO mode while in userland, but linux didn't use to and
 the various *BSDs still don't. Unfortunately the sparc TAS/S_UNLOCK
 were only correct under TSO. Fix that by adding the necessary memory
 barrier instructions. On sparcv8+, which should be all relevant CPUs,
 these are treated as NOPs if the current consistency model doesn't
 require the barriers.
 
 Discussion: 20140630222854.GW26930@awork2.anarazel.de
 
 Will be backpatched to all released branches once a few buildfarm
 cycles haven't shown up problems. As I've no access to sparc, this is
 blindly written.