cmd/compile: make math.Ceil/Floor/Round/Trunc intrinsics on arm64

cmd/compile: make math.Ceil/Floor/Round/Trunc intrinsics on arm64

name       old time/op  new time/op  delta
Ceil        550ns ± 0%   486ns ± 7%  -11.64%  (p=0.000 n=13+18)
Floor       495ns ±19%   512ns ±12%     ~     (p=0.164 n=20+20)
Round       550ns ± 0%   487ns ± 8%  -11.49%  (p=0.000 n=12+19)
Trunc       563ns ± 7%   488ns ±13%  -13.44%  (p=0.000 n=15+2)

Change-Id: I53f234b160b3c026a277506e2cf977d150379464
Reviewed-on: https://go-review.googlesource.com/88295
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>