crypto/tls: update the supported signature algorithms.

crypto/tls: update the supported signature algorithms.

This is the second in a two-part change. See https://golang.org/cl/9415
for details of the overall change.

This change updates the supported signature algorithms to include
SHA-384 and updates all the testdata/ files accordingly. Even some of
the testdata/ files named “TLS1.0” and “TLS1.1” have been updated
because they have TLS 1.2 ClientHello's even though the server picks a
lower version.

Fixes #9757.

Change-Id: Ia76de2b548d3b39cd4aa3f71132b0da7c917debd
Reviewed-on: https://go-review.googlesource.com/9472
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>