sync/atomic: disable hammer pointer tests on wrong size system
sync/atomic: disable hammer pointer tests on wrong size system

hammerCompareAndSwapPointer64 was only passing on
little-endian systems.  hammerCompareAndSwapPointer32 was
writing 8 bytes to a uint32 value on the heap.

R=rsc, dvyukov
CC=golang-dev
http://codereview.appspot.com/5654065

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang