This commit implements atomic operations for the armv7a architecture
using gcc inline assembly.  This offers higher performance compared to
pthread mutexes.

Tested on an am3517 evm, clocked at 600MHz:

./inproc_thr 200 1000000
------------------------
53-60K messages / sec, pthread mutexes
73-90K messages / sec, assembly atomic ops

./inproc_lat 200 1000000
------------------------
average latency: 42.234 [us], pthread mutexes
average latency: 35.496 [us], assembly atomic ops