crypto/x509: store names in signatureAlgorithmDetails.

There is already a table of signature algorithm details so the code
should use it for the name too. This avoids mismatches.

Change-Id: I0d4befbae721ec43db9f87cd93173ec12749e4c8
Reviewed-on: https://go-review.googlesource.com/57210


Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>