crypto/x509: use the platform verifier on iOS

Use the same certificate verification APIs on iOS as on macOS (they
share the same APIs, so we should be able to transparently use them
on both.)

Updates #46287
Fixes #38843

Change-Id: If70f99b0823dd5fa747c42ff4f20c3b625605327
Reviewed-on: https://go-review.googlesource.com/c/go/+/353403


Trust: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>