crypto/x509/internal/macos: simplify code using unsafe.{SliceData,StringData}

crypto/x509/internal/macos: simplify code using unsafe.{SliceData,StringData}

Updates #54854

Change-Id: I8a64a1176cbe16489e1fd21c66a7abc7d8b8e9b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/428154
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>