runtime: simplify code using unsafe.{Slice,String}

runtime: simplify code using unsafe.{Slice,String}

Updates #54854

Change-Id: Ie18665e93e477b6f220acf4c6c070b2af4343064
Reviewed-on: https://go-review.googlesource.com/c/go/+/428157
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>