Add an IGNORE_ENOSYS flag and use it

Some syscalls return ENOSYS depending on their arguments. We don't want
to stop calling them just because we hit one of those cases. Add a flag
to specify this behaviour so we don't have to keep special-casing those
calls in mkcall().
Mark perf_event_open() with IGNORE_ENOSYS

At the very least specifying PERF_SAMPLE_STACK_USER will return ENOSYS
on arches that don't implement it.
Update powerpc syscall list to match v3.15-rc7