[RuntimeDyld] Fix ppc64 stub relocations on little-endian

When RuntimeDyldELF creates stub functions, it needs to install
relocations that will resolve to the final address of the target
routine. Since those are 16-bit relocs, they need to be applied to the
least-significant halfword of the instruction.  On big-endian ppc64,
this means that addresses have to be adjusted by 2, which is what the
code currently does.

However, on a little-endian system, the address must *not* be adjusted;
the least-significant halfword is the first one.  This patch updates the
RuntimeDyldELF code to take the target byte order into account.