ghc treats OSUnknown (and GNU/Hurd) as non-ELF target.
This causes panic in native codegenerator when trying
to build PIC code:

```
  ...
  -- all other platforms
  howToAccessLabel dflags _ _ _ _ _
        | not (positionIndependent dflags)
        = AccessDirectly
        | otherwise
        = panic "howToAccessLabel: PIC not defined for this platform"
```

This change declares new 'OSHurd' and marks it as an
ELF target. Fixes building ghc-stage2 on i686-unknown-gnu0.9.

Patch provided by "Pino" via Samuel Thibault and taken from
debian.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>