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
https://golang.org/cl/5654065

src/pkg/sync/atomic/atomic_test.go

@@ -636,6 +636,7 @@ func init() {
 		// 64-bit system; clear uintptr tests
 		hammer32[2].f = nil
 		hammer32[5].f = nil
+		hammer32[6].f = nil
 	}
 }
 
@@ -760,6 +761,7 @@ func init() {
 		// 32-bit system; clear uintptr tests
 		hammer64[2].f = nil
 		hammer64[5].f = nil
+		hammer64[6].f = nil
 	}
 }