cmd/internal/obj/loong64: add support for FSEL instruction

cmd/internal/obj/loong64: add support for FSEL instruction

Go asm syntax:
	FSEL	FCC, FK, FJ, FD

Equivalent platform assembler syntax:
	fsel	fd, fj, fk, ca

Change-Id: If75f16fca0adfc03f4952f8a5143d22da33ed425
Reviewed-on: https://go-review.googlesource.com/c/go/+/693457
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>