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 <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go