Remove missing archs (mipseb, mipsel, alpha) (#5734)
Remove missing archs (mipseb, mipsel, alpha) (#5734)

It doesn't hurt to map these to ArchUnknown since we don't need to
know anything specific about them, and adding them would be a pain
(there are a bunch of places where we have to case-match on all the
arches to avoid warnings).

ilyasergey/GHC-XAppFix,ilyasergey/GHC-XAppFix,ilyasergey/GHC-XAppFix,ilyasergey/GHC-XAppFix