Comodo are now using a SHA-384 signed intermediate. The crypto/x509
package seeks to import hash functions needed for typical operation
without needing to import every hash function possible. Since a SHA-384
certificate is being used by Comodo, crypto/sha512 now appears to fall
into the scope of "typical operation".

R=bradfitz

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/87670045