internal/abi: define loong64 regABI constants

internal/abi: define loong64 regABI constants

Update #40724

Co-authored-by: Xiaolin Zhao <zhaoxiaolin@loongson.cn>
Change-Id: Id580d9e22a562adee2ae02a467ac38a54949e737
Reviewed-on: https://go-review.googlesource.com/c/go/+/521778
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: David Chase <drchase@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>