Add support for ECDSA P-384 and P-521 curves to XML Signature

XML Signature 1.1 recommends that implementations support ECDSA over the P-384 and P-521 prime curves; see http://www.w3.org/TR/xmldsig-core1/#sec-ECDSA.

We used to support these curves (as well as the required P-256 curve); however the implementation was dependent on internal APIs which changed and inadvertantly broke the behavior. So, we removed the dependency on the internal APIs in JDK-8046724 but we only added support for P-256.

We should also restore the behavior for P-384 and P-521 as technically this is a regression. Not supporting these is not good for interoperability as other vendors already support them; for more details see the XML Signature 1.1 Interop Test Report: http://www.w3.org/TR/xmldsig-core1-interop/