[dev.simd] go/types: exclude simd/_gen module from TestStdlib

We're about to add a small simd/_gen submodule that imports external
dependencies. Exclude it from TestStdlib since it won't be able to
follow those dependencies.

Change-Id: I29a1adc98d141b9c511aa29e1992fab2248747d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/695976
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -358,6 +358,8 @@ func TestStdKen(t *testing.T) {
 var excluded = map[string]bool{
 	"builtin":                       true,
 	"cmd/compile/internal/ssa/_gen": true,
+	"simd/_gen/simdgen":             true,
+	"simd/_gen/unify":               true,
 }
 
 // printPackageMu synchronizes the printing of type-checked package files in

src/go/types/stdlib_test.go

@@ -360,6 +360,8 @@ func TestStdKen(t *testing.T) {
 var excluded = map[string]bool{
 	"builtin":                       true,
 	"cmd/compile/internal/ssa/_gen": true,
+	"simd/_gen/simdgen":             true,
+	"simd/_gen/unify":               true,
 }
 
 // printPackageMu synchronizes the printing of type-checked package files in