cmd/internal/obj/loong64: add support for instructions FSCALEB{F/D} and FLOGB{F/D}

cmd/internal/obj/loong64: add support for instructions FSCALEB{F/D} and FLOGB{F/D}

Go asm syntax:
	FSCALEB{F/D}	FK, FJ, FD
	FLOGB{F/D}	FJ, FD

Equivalent platform assembler syntax:
	fscaleb.{s/d}	fd, fj, fk
	flogb.{s/d}	fd, fj

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

Change-Id: I6cd75c7605adbb572dae86d6470ec7cf20ce0f6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/612975
Auto-Submit: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>