cmd/internal/obj/loong64: add support for instructions FFINT.{S/D}.{W/L} and FTINT...

cmd/internal/obj/loong64: add support for instructions FFINT.{S/D}.{W/L} and FTINT.{W/L}.{S/D}

Go asm syntax:
	FFINT{F/D}{W/V}		FJ, FD
	FTINT{W/V}{F/D}		FJ, FD

Equivalent platform assembler syntax:
	ffint.{s/d}.{w/l}	fd, fj
	ftint.{w/l}.{s/d}	fd, fj

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

Change-Id: Ie7646c5d49645c63b274b34b66539f10370f4930
Reviewed-on: https://go-review.googlesource.com/c/go/+/590996
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>