[dev.simd] simd: add goexperiment tag to generate.go

[dev.simd] simd: add goexperiment tag to generate.go

So the simd package does not exist, instead of existing as an
empty package, if the goexperiment is not enabled. Unfortunately
the simd package developers have to run
GOEXPERIMENT=simd go generate, especially if one is not on an
AMD64 machine. But that command is still simple enough, not too
bad.

Change-Id: I632ce92ecb72e208212e294d8b3448b43fd01eef
Reviewed-on: https://go-review.googlesource.com/c/go/+/723802
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>