runtime/cgo: Don't define crosscall2 in dummy _cgo_main.c.

In this specific package crosscall2 is already defined in a .S
file anyhow.  This avoids a warning about mismatched
alignment.

R=rsc
CC=golang-dev
https://golang.org/cl/4000043

src/pkg/runtime/cgo/Makefile

@@ -48,7 +48,6 @@ _cgo_defun.c:
 
 _cgo_main.c:
 	echo 'int main() { return 0; }' >$@
-	echo 'void *crosscall2;' >>$@
 endif
 
 $(GOARCH).o: $(GOARCH).S