[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.

This changes the PowerPC back-end ABI code to only add the small
argument stack slot offset for BE.  It also adds test cases to verify
the correct behavior on both BE and LE.