initial draft of PPCMachObjectWriter.cpp
this records relocation entries in the mach-o object file
for PIC code generation.
tested on powerpc-darwin8, validated against darwin otool -rvV

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188004 91177308-0d34-0410-b5e6-96231b3b80d8