[dev.typeparams] cmd/compile: define ARM64 parameter registers

Define the registers.

They are not really enabled for now. Otherwise the compiler will
start using them for go:registerparams functions and it is not
fully working. Some test will fail.

Now we can compile a simple Add function with registerparams
(with registers enabled).

Change-Id: Ifdfac931052c0196096a1dd8b0687b5fdedb14d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/322850


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>