acmart: avoid mathabx '\bigtimes'

The 'acmart' class includes a '\bigtimes' command from the 'newtxmath'
package, if available. Scribble includes a '\bigtimes' command from the
'mathabx' package. These cannot co-exist.

If `newtxmath` is available, this PR does not import `mathabx` in acmart
documents.

If `newtxmath` is not available, this PR includes `mathabx` like normal
(same as all previous versions of `scribble/acmart`)

scribble-lib/scribble/acmart/acmart-load.tex

@@ -3,6 +3,8 @@
 % Avoid package option conflict
 \renewcommand\packageColor\relax
 \renewcommand\packageTocstyle\relax
+\renewcommand\packageMathabx{\ifx\bigtimes\undefined \usepackage{mathabx} \else \relax \fi}
+% Both 'mathabx' and 'newtxmath' (required by the 'acmart' class) define a '\bigtimes' command. 
 \let\Footnote\undefined
 \let\captionwidth\undefined
 \renewcommand{\renewrmdefault}{}