[RuntimeDyld] Teach RuntimeDyldMachO how to handle scattered VANILLA relocs on
i386.

This fixes two more MCJIT regression tests on i386:

  ExecutionEngine/MCJIT/2003-05-06-LivenessClobber.ll
  ExecutionEngine/MCJIT/2013-04-04-RelocAddend.ll

The implementation of processScatteredVANILLA is tasteless (*ba-dum-ching*),
but I'm working on a substantial tidy-up of RuntimeDyldMachO that should
improve things.

This patch also fixes a type-o in RuntimeDyldMachO::processSECTDIFFRelocation,
and teaches that method to skip over the PAIR reloc following the SECTDIFF.

<rdar://problem/16961886>