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

apparentlymart/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme