[PowerPC] Fix small argument stack slot offset for LE

When small arguments (structures < 8 bytes or "float") are passed in a
stack slot in the ppc64 SVR4 ABI, they must reside in the least
significant part of that slot.  On BE, this means that an offset needs
to be added to the stack address of the parameter, but on LE, the least
significant part of the slot has the same address as the slot itself.

For the most part, this is handled in the LLVM back-end, where I just
fixed the LE case in commit r211368.

However, there is one piece of the clang front-end that is also aware of
these stack-slot offsets: PPC64_SVR4_ABIInfo::EmitVAArg.  This patch
updates that routine to take endianness into account.

llvm-svn: 211370