[CVE-2009-0029] powerpc: Enable syscall wrappers for 64-bit

This enables the use of syscall wrappers to do proper sign extension
for 64-bit programs.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>