cgo: fix _cgo_run timestamp file order

The timestamp file is being created before cgo
runs, which means errors will not prevent it
from being created and thus will cause the
build to break by rendering the rule up-to-date
when it isn't.

R=rsc
CC=golang-dev
https://golang.org/cl/4001049

src/Make.pkg

@@ -113,8 +113,8 @@ dir:
 
 ifdef CGOFILES
 _cgo_run: $(CGOFILES)
-	@touch _cgo_run
 	CGOPKGPATH=$(dir) cgo -- $(CGO_CFLAGS) $(CGOFILES)
+	touch _cgo_run
 
 # _CGO_CFLAGS and _CGO_LDFLAGS are defined via the evaluation of _cgo_flags.
 # The include happens before the commands in the recipe run,