[dev.simd] simd: refactor big template in gen_simdTypes into map

This does not change the generated code

Change-Id: I10082b406cc023f42ca5e260e923da08909d7c4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/779440
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: David Chase <drchase@google.com>