This is the equivalent change to 1c105980 but for SHA-512.

SHA-512 certificates are already supported by default since b53bb2ca,
but some servers will refuse connections if the algorithm is not
advertised in the overloaded signatureAndHash extension (see 09b238f1).

This required adding support for SHA-512 signatures on CertificateVerify
and ServerKeyExchange messages, because of said overloading.

Some testdata/Client-TLSv1{0,1} files changed because they send a 1.2
ClientHello even if the server picks a lower version.

Closes #22422

Change-Id: I16282d03a3040260d203711ec21e6b20a0e1e105
Reviewed-on: https://go-review.googlesource.com/74950
Run-TryBot: Filippo Valsorda <hi@filippo.io>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>