[RelocationResolver] Support R_PPC_REL32 & R_PPC64_REL{32,64}

This suppresses `failed to compute relocation: R_PPC_REL32, Invalid data was encountered while parsing the file`
and its 64-bit variants when running llvm-dwarfdump on a PowerPC object file with .eh_frame

Unfortunately it is difficult to test the computation:
DWARFDataExtractor::getEncodedPointer does not use the relocated value
and even if it does, we need to teach llvm-dwarfdump --eh-frame to do
some linker job to report a reasonable address.

(cherry picked from commit b922004e)