mkaurball: pass --ignorearch to makepkg

FIXME: we pass --ignorearch because makepkg will refuse to make
a source package for an architecture other than one supported by
the PKGBUILD. This makes no sense since source tarball creation
is an architecture independent process.
regtest: diff against origin/master by default
add proper support for arch specific attributes

makepkg emits arch-specific attributes as of pacman git commit 2b556d89.
Patches to extend this to support the source array are in flight, so
this patch here adds arch-specific sources.
fix broken arch-specific handling