cmd/asm, cmd/internal/obj: enable rounding mode suffix for riscv64

cmd/asm, cmd/internal/obj: enable rounding mode suffix for riscv64

This CL adds rounding modes for riscv64 floating point conversion
instructions by suffix with 5 modes: RNE, RTZ, RDN, RUP and RMM.

For example, for round to nearest (RNE), we can use `FCVTLD.RNE`
According to RISCV manual 8.7 and 9.5, we changed these
conversion instructions:

FCVTWS
FCVTLS
FCVTWUS
FCVTLUS
FCVTWD
FCVTLD
FCVTWUD
FCVTLUD

Note: Round towards zero (RTZ) by default for all these instructions above.

Change-Id: I491e522e14d721e24aa7f528ee0c4640c54c5808
Reviewed-on: https://go-review.googlesource.com/c/go/+/504736
Reviewed-by: Joel Sing <joel@sing.id.au>
Run-TryBot: M Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>