cmd/internal/obj/loong64, cmd/internal/objabi, cmd/link: add support for --buildmode=c-shared on loong64

Updates #53301
Updates #58784

Change-Id: Ifcb40871f609531dfd8b568db9ac14da9b451742
Reviewed-on: https://go-review.googlesource.com/c/go/+/425476


Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Run-TryBot: WANG Xuerui <git@xen0n.name>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: WANG Xuerui <git@xen0n.name>