Add \hypertarget to enable url links into xrefs

[t3nsor]

Fixes #6510 .

[t3nsor]

I've built the PDF locally and tested that the following kinds of URL hash fragments do the right thing:

• #dcl.init.general: goes to [dcl.init.general] in the main text
• #defns.static.type: goes to the definition of "static type"
• #implimits: goes to Annex B.

It was necessary to move the \addxref command to the beginning of the expansions that it's referenced in, to ensure that the \hypertarget targets the end of the previous line, not the end of the line that the heading is located on. So I've also checked that the glossary entries still go to the right place. It appears that they do.