sim: ppc: implement 128-bit register read/writes with sim-endian APIs

sim: ppc: implement 128-bit register read/writes with sim-endian APIs

We have APIs in sim-endian for working with 128-bit values like this code
is already doing for 8/16/32/64-bit values.  Switch over to that to make
it a bit simpler, and drop the WITH_ALTIVEC check.  The code probably is
only used when altivec is enabled, but it doesn't add much to always
compile it in, and avoids #ifdef rot by not actually compiling it.