[dev.simd] simd, cmd/compile, go build, go/doc: test tweaks

[dev.simd] simd, cmd/compile, go build, go/doc: test tweaks

these are for CL 675618
simd package exists and imports internal/cpu
tweak tests to deal with goexperiment/not

Change-Id: I2de99d048f0a228d5f3cd750c39ee5925107556e
Reviewed-on: https://go-review.googlesource.com/c/go/+/677260
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Junyang Shao <shaojunyang@google.com>