runtime: use unsafe.Slice in getStackMap

runtime: use unsafe.Slice in getStackMap

CL 362934 added open code for unsafe.Slice, so using it now no longer
negatively impacts the performance.

Updates #48798

Change-Id: Ifbabe8bc1cc4349c5bcd11586a11fc99bcb388b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/404974
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>