plugins: add magic values for all previously supported ABIs (and a couple of new ones) to mumble_plugin.h.

Add all previous magic values we've used since at least 1.2.0.
(Though I think the MUMBLE_PLUGIN_MAGIC value for MSVS2008 x86
goes back even further!)

This also adds a new magic value for the MSVS2013 x64 ABI, and
a generic default for "unsupported" ABIs.

Unsupported in this context means platforms/ABIs that we aren't
distributing plugins for via the auto-update mechanism. Those
platforms typically only have a single plugin, which is the
"link" plugin distributed with Mumble itself.