* Represent backends with a `Backend` datatype in GHC.Driver.Backend

* Don't detect the default backend to use for the target platform at
  compile time in Hadrian/make but at runtime. It makes "Settings"
  simpler and it is a step toward making GHC multi-target.

* The latter change also fixes hadrian which has not been updated to
  take into account that the NCG now supports AIX and PPC64 (cf
  df26b955 and
  d3c1dda6)

* Also we don't treat iOS specifically anymore (cf
  cb4878ff)