cmd/compile: intrinsify math.RoundToEven and math.Abs on arm64

math.RoundToEven can be done by one arm64 instruction FRINTND, intrinsify it to improve performance.
The current pure Go implementation of the function Abs is translated into five instructions on arm64:
str, ldr, and, str, ldr. The intrinsic implementation requires only one instruction, so in terms of
performance, intrinsify it is worthwhile.

Benchmarks:
name           old time/op  new time/op  delta
Abs-8          3.50ns ± 0%  1.50ns ± 0%  -57.14%  (p=0.000 n=10+10)
RoundToEven-8  9.26ns ± 0%  1.50ns ± 0%  -83.80%  (p=0.000 n=10+10)

Change-Id: I9456b26ab282b544dfac0154fc86f17aed96ac3d
Reviewed-on: https://go-review.googlesource.com/116535


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>