Implement MipsABIInfo::EmitVAArg. This fix enables clang to complete compilation

without bailing out when va_arg is an aggregate expression. However, 
alignment checking needs to be added in isSafeToEliminateVarargsCast in
InstCombineCalls.cpp in order to produce correct mips code (see link below).

http://lists.cs.uiuc.edu/pipermail/llvmdev/2011-July/042047.html

llvm-svn: 136647