x86_64 is used as a baseline: the "x86_64" entry, whatever status it has,
is transformed into "all", and then the other entries in ARCHITECTURES
either dropped or rearranged appropriately.
* Add x86 and x86_64 to ARCHITECTURES as the recipe also works fine for these.
* Add x86_gcc2 to SECONDARY_ARCHITECTURES for use in x86 (gcc4) hybrid.
* Use $portVersion instead of the current version in SOURCE_{URI,DIR}.
* Drop the "r" prefix in version (r475 → 475) and ajust SOURCE_{URI,DIR}.
* Referring the current haiku version explicitly is not needed, since
the RequiresUpdater takes care of setting the version of Haiku used
for building a package.
