[dev.simd] cmd/compile: add simdGen prog writer

[dev.simd] cmd/compile: add simdGen prog writer

This CL is a synergy between simdgen refactor CL 681195.

Change-Id: I365becf515a261bd22c46824613c2dce309cac45
Reviewed-on: https://go-review.googlesource.com/c/go/+/681036
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>