macho gas support

[Alan Modra]

ix86-darwin is the only remaining macho target supported by gas.  I'm
not sure how much it is used nowadays, and judging by the number of
x86 tests and mach-o tests that fail, it seems no one is actively
maintaining the code.  So it's quite possible this series of patches I
made fixing some issues was wasted time.  My interest in the port
started with seeing a regression from commit cc28c46227cd.  Fixing
that led to finding more problems.

Of the three patches, I'm less confident in the correctness of the
last one.  I do know that it is necessary to ignore subsections in at
least obj_mach_o_force_reloc_sub_same, otherwise even the simplest
assembly testcases won't assemble with -gdwarf2, but there may be some
other situation where it is wrong to ignore subsections when
processing debug info.

I haven't tried to build gcc for i386-darwin, if that is even
possible.

-- 
Alan Modra