cmd/compile: teach regalloc to rightly do nothing on loong64 in case of dynlinking

cmd/compile: teach regalloc to rightly do nothing on loong64 in case of dynlinking

This is needed before actual support for buildmode=plugin is added.
Should not affect current behavior.

Change-Id: I86371d7e373fd529cb8710850d7b0fbbf1eb52ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/480877
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: WANG Xuerui <git@xen0n.name>
TryBot-Result: Gopher Robot <gobot@golang.org>