cmd/internal/obj/loong64: add support for instructions FTINT{RM/RP/RZ/RNE}.{W/L}...

cmd/internal/obj/loong64: add support for instructions FTINT{RM/RP/RZ/RNE}.{W/L}.{S/D}

These instructions convert floating-point numbers to fixed-point numbers
with the specified rounding pattern.

Go asm syntax:
            FTINT{RM/RP/RZ/RNE}{W/V}{F/D}	FJ, FD

Equivalent platform assembler syntax:
            ftint{rm/rp/rz/rne}.{w/l}.{s/d}	fd, fj

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

Change-Id: I6d650d1b48b10296d01a98fadf9d806206f9b96e
Reviewed-on: https://go-review.googlesource.com/c/go/+/590995
Auto-Submit: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>