cmd/compile: reorganize compiler type descriptor generation

cmd/compile: reorganize compiler type descriptor generation

Use the new rttype mechanism to share internal/abi types between
the compiler and runtime.

Change-Id: I2bbba4d8090c6f7ff20dca15b7b665f5d04e5bfd
Reviewed-on: https://go-review.googlesource.com/c/go/+/534936
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: David Chase <drchase@google.com>