System V ABI support for the PowerPC backend

by Tilmann for LLVM Compiler Infrastructure

The goal of this project is to implement support for the System V ABI in the PowerPC backend of LLVM. Since this is the ABI used on 32-bit PowerPC Linux, this will effectively bring LLVM to the 32-bit PowerPC Linux platform. At the moment PowerPC Linux is not officially supported by LLVM, however the PowerPC backend is considered to be stable on Darwin.