ppc sim: Allow --sysroot command-line option

ppc sim: Allow --sysroot command-line option

There was a recent change that cuased the "target sim" command
to add a --sysroot option to the argument vector passed down to
the simulator.  This caused a failure in the powerpc simulator,
as it did not recognize it.  This patch fixes the problem by adding
support for the --sysroot option (it ignores it).

sim/ppc/ChangeLog:

        * psim.c (psim_options): Accept and ignore `--sysroot=...'.