Preserve xref macro syntax to ensure text and attributes are parsed properly

It was discovered that xrefs to pages with the xrefstyle set are not rendering correctly in the latest Assembler Beta.8.

There is an error in translation of the xref macro. The shorthand does not support attributes, so they are treated as text.

chapter-1.adoc

= Title of Chapter 1

xref:chapter-1.adoc[xrefstyle=short]

This results in <<chapter-1,xrefstyle=short>>