[PowerPC] Use the ABI indirect-call protocol for patchpoints

We used to take the address specified as the direct target of the patchpoint
and did no TOC-pointer handling.  This, however, as not all that useful,
because MCJIT tends to create a lot of modules, and they have their own TOC
sections. Thus, to call from the generated code to other generated code, you
really need to switch TOC pointers. Make this work as expected, and under
ELFv1, tread the address as the function descriptor address so that the correct
TOC pointer can be loaded.

llvm-svn: 242217