Hardware AES support in Go on s390x currently requires ECB, CBC
and CTR modes be available. It also requires that either the
GHASH or GCM facilities are available. The existing checks missed
some of these constraints.

While we're here simplify the cpu package on s390x, moving masking
code out of assembly and into Go code. Also, update SHA-{1,256,512}
implementations to use the cpu package since that is now trivial.

Finally I also added a test for internal/cpu on s390x which loads
/proc/cpuinfo and checks it against the flags set by internal/cpu.

Updates #25822 for changes to vet whitelist.

Change-Id: Iac4183f571643209e027f730989c60a811c928eb
Reviewed-on: https://go-review.googlesource.com/114397
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>