math: add round assembly implementations on riscv64

math: add round assembly implementations on riscv64

goos: linux
goarch: riscv64
pkg: math
            │ floor_old.bench │           floor_new.bench           │
            │     sec/op      │   sec/op     vs base                │
Ceil              54.12n ± 0%   22.05n ± 0%  -59.26% (p=0.000 n=10)
Floor             40.80n ± 0%   22.05n ± 0%  -45.96% (p=0.000 n=10)
Round             20.73n ± 0%   20.74n ± 0%        ~ (p=0.441 n=10)
RoundToEven       24.07n ± 0%   24.07n ± 0%        ~ (p=1.000 n=10)
Trunc             38.73n ± 0%   22.05n ± 0%  -43.07% (p=0.000 n=10)
geomean           33.58n        22.17n       -33.98%

Change-Id: I24fb9e3bbf8146da253b6791b21377bea1afbd16
Reviewed-on: https://go-review.googlesource.com/c/go/+/504737
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: M Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: M Zhuo <mengzhuo1203@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>