[PowerPC] Fix regression in generating @ha/@l relocs

The patch I committed as revision 167864 introduced a regression that
causes LLVM to no longer generate appropriate relocs for @ha/@l symbol
references (but fail an assertion instead).

This is fixed here by re-enabling support for the VK_PPC_GAS_HA16/
VK_PPC_GAS_LO16 variant kinds (and their Darwin variants) in
PPCELFObjectWriter.cpp.

Tested by running projects/test-suite in -m32 mode with the integrated
assembler forced on.  A standalone test case will be committed shortly
as well.