This CL instrinsifies Add64 with arm64 instruction sequence ADDS, ADCS
and ADC, and optimzes the case of carry chains.The CL also changes the
test code so that the intrinsic implementation can be tested.

Benchmarks:
name               old time/op       new time/op       delta
Add-224            2.500000ns +- 0%  2.090000ns +- 4%  -16.40%  (p=0.000 n=9+10)
Add32-224          2.500000ns +- 0%  2.500000ns +- 0%     ~     (all equal)
Add64-224          2.500000ns +- 0%  1.577778ns +- 2%  -36.89%  (p=0.000 n=10+9)
Add64multiple-224  6.000000ns +- 0%  2.000000ns +- 0%  -66.67%  (p=0.000 n=10+10)

Change-Id: I6ee91c9a85c16cc72ade5fd94868c579f16c7615
Reviewed-on: https://go-review.googlesource.com/c/go/+/159017
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>