cmd/go: issue33139 is only valid when running with CGO

With CGO enabled, the test suite tries to run the following and fail:
cc -c -o syso/objTestImpl.syso syso/src/objTestImpl.c

Author: nehaljwani

src/cmd/go/testdata/script/link_syso_issue33139.txt

@@ -3,6 +3,7 @@
 # See issue 33139.
 [!gc] skip
 [!exec:cc] skip
+[!cgo] skip
 
 # External linking is not supported on linux/ppc64.
 # See: https://github.com/golang/go/issues/8912