Fix handling of mach header and DSO handle symbols.

The magic file which contained these symbols inherited from archive
which meant that the resolver didn't add the required atoms as archive
members only get added when referenced.  Instead we now inherit from
SimpleFile which always links in the atoms needed.

The second issue was in the handling of these symbols when we emit
the MachO.  The mach header symbol needs to be in the atom list as
it gets an offset (0), and being in the atom list makes sure it is
emitted to the symbol table.  DSO handles are not emitted to the
symbol table.

rdar://problem/24450654

llvm-svn: 259574