[PowerPC] Generate little-endian object files

As a first step towards real little-endian code generation, this patch
changes the PowerPC MC layer to actually generate little-endian object
files.  This involves passing the little-endian flag through the various
layers, including down to createELFObjectWriter so we actually get basic
little-endian ELF objects, emitting instructions in little-endian order,
and handling fixups and relocations as appropriate for little-endian.

The bulk of the patch is to update most test cases in test/MC/PowerPC
to verify both big- and little-endian encodings.  (The only test cases
*not* updated are those that create actual big-endian ABI code, like
the TLS tests.)

Note that while the object files are now little-endian, the generated
code itself is not yet updated, in particular, it still does not adhere
to the ELFv2 ABI.