[mach-o]: support -syslibroot and -l options

These behave slightly idiosyncratically in the best of cases, and have
additional hacks layered on top of that for compatibility with badly behaved
build systems (via ld64).

For -lXYZ:
  + If XYZ is actually XY.o then search all library paths for XY.o
  + Otherwise search all library paths, first for libXYZ.dylib, then libXYZ.a
  + By default the library paths are /usr/lib and /usr/local/lib in that order.

For -syslibroot:
  + -syslibroot options apply to absolute paths in the search order.
  + All -syslibroot prefixes that exist are added to the search path *instead*
    of the original.
  + If no -syslibroot prefixed path exists, the original is kept.
  + Hacks^WExceptions:
      + If only 1 -syslibroot is given and doesn't contain /usr/lib or
        /usr/local/lib, that path is dropped entirely. (rdar://problem/6438270).
      + If the last -syslibroot is "/", all of them are ignored entirely.
        (rdar://problem/5829579).

At least, that's my best interpretation of what ld64 does in buildSearchPaths.