internal/simdgen: replace go generate with !import

Currently, after editing the source YAML files, you have to run go
generate to produce that concatenated inputs. This is easy to forget
to do, and it's easy to accidentally edit the concatenated input
instead of the source YAML. It also means any line numbers that appear
in error messages or debug output refer to the generated YAML instead
of the source YAML.

Fix this by using the new !import node to perform this file loading in
the unifier itself from the original source files.

Change-Id: I735f96ac9b12ccd1057629758a73b1bda30544cc
Reviewed-on: https://go-review.googlesource.com/c/arch/+/693344
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>