[lld-macho][nfc] Refactor in preparation for 32-bit support

The main challenge was handling the different on-disk structures (e.g.
`mach_header` vs `mach_header_64`). I tried to strike a balance between
sprinkling `target->wordSize == 8` checks everywhere (branchy = slow, and ugly)
and templatizing everything (causes code bloat, also ugly). I think I struck a
decent balance by judicious use of type erasure.

Note that LLD-ELF has a similar architecture, though it seems to use more templating.

Linking chromium_framework takes about the same time before and after this
change:

      N           Min           Max        Median           Avg        Stddev
  x  20          4.52          4.67         4.595        4.5945   0.044423204
  +  20           4.5          4.71         4.575         4.582   0.056344803
  No difference proven at 95.0% confidence

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D99633