[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>