Revert "runtime: use unsafe.Slice in getStackMap"

Revert "runtime: use unsafe.Slice in getStackMap"

This reverts commit golang.org/cl/352953.

Reason for revert: unsafe.Slice is considerably slower.
Part of this is extra safety checks (good), but most of it
is the function call overhead. We should consider open-coding it (#48798).

Impact of this change:

name                   old time/op  new time/op  delta
StackCopyWithStkobj-8  12.1ms ± 5%  11.6ms ± 3%  -4.03%  (p=0.009 n=10+8)

Change-Id: Ib2448e3edac25afd8fb55ffbea073b8b11521bde
Reviewed-on: https://go-review.googlesource.com/c/go/+/354090
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>