sync/atomic: add more missing Go prototype

Should fix nacl/arm build.

TBR=minux
CC=golang-codereviews
https://golang.org/cl/145070043

src/sync/atomic/64bit_arm.go

@@ -50,3 +50,9 @@ func swapUint64(addr *uint64, new uint64) (old uint64) {
 func armCompareAndSwapUint32(addr *uint32, old, new uint32) (swapped bool)
 func armCompareAndSwapUint64(addr *uint64, old, new uint64) (swapped bool)
 func generalCAS64(addr *uint64, old, new uint64) (swapped bool)
+func armAddUint32(addr *uint32, delta uint32) (new uint32)
+func armAddUint64(addr *uint64, delta uint64) (new uint64)
+func armSwapUint32(addr *uint32, new uint32) (old uint32)
+func armSwapUint64(addr *uint64, new uint64) (old uint64)
+func armLoadUint64(addr *uint64) (val uint64)
+func armStoreUint64(addr *uint64, val uint64)