runtime: add fast version of getArgInfo

getArgInfo is called a lot during stack copying.
In the common case it doesn't do much work,
but it cannot be inlined.

This change works around that.

name                old time/op  new time/op  delta
StackCopyPtr-8       108ms ± 5%    96ms ± 4%  -10.40%  (p=0.000 n=20+20)
StackCopy-8         82.6ms ± 3%  78.4ms ± 6%   -5.15%  (p=0.000 n=19+20)
StackCopyNoCache-8   130ms ± 3%   122ms ± 3%   -6.44%  (p=0.000 n=20+20)

Change-Id: If7d8a08c50a4e2e76e4331b399396c5dbe88c2ce
Reviewed-on: https://go-review.googlesource.com/108945


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>