Optimize arm64 CALLFN wrapper with LDP/STP instructions.
This provides a significant speedup for big argument copy.
Benchmark results for reflect:

name                      old time/op    new time/op     delta
Call-8                      79.0ns ± 4%     73.6ns ± 4%    -6.78%  (p=0.000 n=10+10)
CallArgCopy/size=128-8      80.5ns ± 0%     60.3ns ± 0%   -25.06%  (p=0.000 n=10+9)
CallArgCopy/size=256-8       119ns ± 2%       67ns ± 1%   -43.59%  (p=0.000 n=8+10)
CallArgCopy/size=1024-8      524ns ± 1%       99ns ± 1%   -81.03%  (p=0.000 n=10+10)
CallArgCopy/size=4096-8      837ns ± 0%      231ns ± 1%   -72.42%  (p=0.000 n=9+9)
CallArgCopy/size=65536-8    13.6µs ± 6%      3.1µs ± 1%   -77.38%  (p=0.000 n=10+10)
PtrTo-8                     12.9ns ± 0%     13.1ns ± 3%    +1.86%  (p=0.000 n=10+10)
FieldByName1-8              28.7ns ± 2%     28.6ns ± 2%      ~     (p=0.408 n=9+10)
FieldByName2-8               928ns ± 4%      946ns ± 8%      ~     (p=0.326 n=9+10)
FieldByName3-8              5.35µs ± 5%     5.32µs ± 5%      ~     (p=0.755 n=10+10)
InterfaceBig-8              2.57ns ± 0%     2.57ns ± 0%      ~     (all equal)
InterfaceSmall-8            2.57ns ± 0%     2.57ns ± 0%      ~     (all equal)
New-8                       9.09ns ± 1%     8.83ns ± 1%    -2.81%  (p=0.000 n=10+9)

name                      old alloc/op   new alloc/op    delta
Call-8                       0.00B           0.00B           ~     (all equal)

name                      old allocs/op  new allocs/op   delta
Call-8                        0.00            0.00           ~     (all equal)

name                      old speed      new speed       delta
CallArgCopy/size=128-8    1.59GB/s ± 0%   2.12GB/s ± 1%   +33.46%  (p=0.000 n=10+9)
CallArgCopy/size=256-8    2.14GB/s ± 2%   3.81GB/s ± 1%   +78.02%  (p=0.000 n=8+10)
CallArgCopy/size=1024-8   1.95GB/s ± 1%  10.30GB/s ± 0%  +427.99%  (p=0.000 n=10+9)
CallArgCopy/size=4096-8   4.89GB/s ± 0%  17.69GB/s ± 1%  +261.87%  (p=0.000 n=9+9)
CallArgCopy/size=65536-8  4.84GB/s ± 6%  21.36GB/s ± 1%  +341.67%  (p=0.000 n=10+10)

Change-Id: I775d88b30c43cb2eda1d0612ac15e6d283e70beb
Reviewed-on: https://go-review.googlesource.com/70570Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>