crypto/x509: tidy darwin/arm64 build tags

The cgo build tag is not necessary for root_darwin_arm64.go. We can't
build for darwin/arm64 without cgo, and even if we did 1) this code
would work fine 2) the no-cgo code that shells out to
/usr/bin/security would not work.

(Suggested by Filippo.)

Change-Id: I98cac2ea96ec5ac1ae60b7e32d195d5e86e2bd66
Reviewed-on: https://go-review.googlesource.com/c/go/+/227583


Reviewed-by: Filippo Valsorda <filippo@golang.org>