cmd/internal/obj/loong64: add support for instructions FCOPYSIGN.{S/D} and FCLASS...

cmd/internal/obj/loong64: add support for instructions FCOPYSIGN.{S/D} and FCLASS.{S/D}

Go asm syntax:
	FCOPYSG{F/D}	FK, FJ, FD
	FCLASSF{F/D}	FJ, FD

Equivalent platform assembler syntax:
	fcopysign.{s/d}	fd, fj, fk
	fclass.{s/d}	fd, fj

Ref: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html

Change-Id: Ied34b71c9d0b34456ac5782a59d29d2d0229e326
Reviewed-on: https://go-review.googlesource.com/c/go/+/590675
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>