[POWERPC] Fix PPC32 SECCOMP, unexport do_syscall_trace_{enter,leave}

The secure_computing() call which automatically aborts a process if it
tries to execute a syscall it shouldn't is much more useful if we
actually do it _before_ the syscall, rather than afterwards. PPC64 got
this right, but the original incorrect behaviour inherited from arch/ppc
was preserved by ifdefs. Make it the same on PPC32 too.

Also, I see no need to export do_syscall_trace_{leave,enter} on ppc32 --
they were only exported because the old do_syscall_trace() (which they
replaced) used to be.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>