crypto/x509/internal/macos: use APIs available on ios

Use SecCertificateCopyData instead of SecItemExport, which is only
available on macOS.

Updates #49616

Change-Id: Ieda33894930d23c6dab6112ee18120f8a440083b
Reviewed-on: https://go-review.googlesource.com/c/go/+/364554


Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>