cmd/compile: add up-to-date test for generated files

cmd/compile: add up-to-date test for generated files

This runs the ssa/_gen generator writing files into
a temporary directory, and then checks that there are
no differences with what is currently in the ssa directory,
and also checks that any file with the "generated from
_gen/..." header was actually generated, and checks that
the headers on the generated file match the expected
header prefix.

Change-Id: Ic8eeb0b06cf6f2e576a013e865b331a12d3a77aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/680615
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>