cmd/compile/internal/ssa: make _gen an actual submodule and skip it in TestStdlib
After tools CL 612038, the package astutil stops being vendored, but
_gen/rulegen.go needs to import this package.
In particular, after update golang.org/x/tools, the package astutil
is deleted from the vendor directory, and got error when run TestStdlib
in longtest. So in this CL, we make _gen an actual submodule and
skip it in TestStdlib.
Change-Id: I76f77b66427f6490b4746698711a6e307ad2ba79
Reviewed-on: https://go-review.googlesource.com/c/go/+/629015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: abner chenc <chenguoqi@loongson.cn>