crypto/x509: Support MD5 signatures for certificates

[gonzojive]

x509.Certificate.CheckSignature does not support MD5 signatures. Is this intentional? If so, a comment should be added in the source and/or docs. If not, then please add support for MD5 signatures.

Code pointer: https://github.com/golang/go/blob/master/src/crypto/x509/x509.go#L621

[ianlancetaylor]

CC @agl

[mvanotti]

I think this is a duplicate of issue #10431