sim: ppc: drop host endian configure option

sim: ppc: drop host endian configure option

The --enable-sim-hostendian flag was purely so people had an escape route
for when cross-compiling.  This is because historically, AC_C_BIGENDIAN
did not work in those cases.  That was fixed a while ago though, so we can
require that macro everywhere now and simplify a good bit of code.

This was done for all the other ports years ago, so catch ppc up.