cmd/internal/obj/loong64: add F{MAXA/MINA}.{S/D} instructions

cmd/internal/obj/loong64: add F{MAXA/MINA}.{S/D} instructions

Go asm syntax:
	F{MAXA/MINA}{F/D}	FK, FJ, FD

Equivalent platform assembler syntax:
	f{maxa/mina}.{s/d}	fd, fj, fk

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

Change-Id: I6790657d2f36bdf5e6818b6c0aaa48117e782b8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/653915
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>