runtime/cgo: silence warning on windows

It appears to want a * on an indirect function call (assembly language)

TBR=rsc

R=golang-dev
CC=golang-dev
https://golang.org/cl/5643054

src/pkg/runtime/cgo/gcc_amd64.S

@@ -70,7 +70,7 @@ EXT(crosscall2):
 	movq  %rdx, 0(%rsp)	/* arg */
 	movq  %r8, 8(%rsp)	/* argsize (includes padding) */
 	
-	call %rcx	/* fn */
+	call *%rcx	/* fn */
 #else
 	movq  %rsi, 0(%rsp)	/* arg */
 	movq  %rdx, 8(%rsp)	/* argsize (includes padding) */