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>

src/cmd/compile/internal/ssa/_gen/go.mod

@@ -0,0 +1,5 @@
+module _gen
+
+go 1.24
+
+require golang.org/x/tools v0.27.0

src/cmd/compile/internal/ssa/_gen/go.sum

@@ -0,0 +1,2 @@
+golang.org/x/tools v0.27.0 h1:qEKojBykQkQ4EynWy4S8Weg69NumxKdn40Fce3uc/8o=
+golang.org/x/tools v0.27.0/go.mod h1:sUi0ZgbwW9ZPAq26Ekut+weQPR5eIM6GQLQ1Yjm1H0Q=

src/cmd/compile/internal/types2/stdlib_test.go

@@ -355,6 +355,7 @@ func TestStdKen(t *testing.T) {
 // Package paths of excluded packages.
 var excluded = map[string]bool{
 	"builtin":                       true,
+	"cmd/compile/internal/ssa/_gen": true,
 }
 
 // printPackageMu synchronizes the printing of type-checked package files in

src/go/types/stdlib_test.go

@@ -357,6 +357,7 @@ func TestStdKen(t *testing.T) {
 // Package paths of excluded packages.
 var excluded = map[string]bool{
 	"builtin":                       true,
+	"cmd/compile/internal/ssa/_gen": true,
 }
 
 // printPackageMu synchronizes the printing of type-checked package files in