sim: ppc: workaround uninitialized variable compiler warnings

sim: ppc: workaround uninitialized variable compiler warnings

Some compilers don't understand the semctl API and think it's an input
argument even when it's used as an output, and then complains that it
is being used uninitialized.  Zero it out explicitly to workaround it.
This adds some runtime overhead, but should be fairly minor as it's a
small stack buffer, and shouldn't be that relevant relative to all the
other logic in these functions.