First pass at implementation of load/store exclusive and 
load/store exclusive w/ read-acquire/store-release:
LD{,A}X{R,RH,RB}
ST{,L}X{R,RH,RB}



git-svn-id: svn://svn.valgrind.org/vex/trunk@2823 8f6e269a-dfd6-0310-a8e1-e2731360e62c