[dev.simd] go/types, cmd/compile: exclude simd/.../sgutil from TestStdlib

This was failing on some longtest builders.
And it is certainly the intent of the two excludes lists
that this directory be there.

For #79677

Change-Id: I2cc90983be0737b29a671e6304b289ca6426775d
Reviewed-on: https://go-review.googlesource.com/c/go/+/783921
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>