aclocal.m4: add OSHurd (debian patch)

Authored by trofi on Mar 17 2018, 12:12 PM.

Description

aclocal.m4: add OSHurd (debian patch)

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>