sync: fix linux/arm build

For real this time.

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://golang.org/cl/141640043

src/sync/atomic/export_linux_arm_test.go

@@ -4,6 +4,4 @@
 
 package atomic
 
-func generalCAS64(addr *uint64, old uint64, new uint64) bool
-
 var GeneralCAS64 = generalCAS64