8037259: xerces update: xpointer update

[Elliott Baron]

Hi,

I'm requesting a backport of 8037259 to 8u, as part of an update of 
Xerces to 2.12.0. Xerces should be updated for parity with Oracle JDK.

The JDK 9 fix applies cleanly except for piece of context in 
ElementSchemePointer, caused by "8048021: Remove @version tag in jaxp repo".

Bug: https://bugs.openjdk.java.net/browse/JDK-8037259
8u Webrev: https://cr.openjdk.java.net/~ebaron/jdk8u/JDK-8037259/webrev.00/

Testing: x86_64 build, jdk_tier1, jdk_other (includes javax/xml) tests

Thanks,
Elliott