powerpc: Wire up statx() syscall

Test runs on a ppc64 BE guest succeeded. linux/samples/statx/test-statx
program was executed on the following file types,

1. Regular file
2. Directory
3. device file
4. symlink
5. Named pipe

The test run also included invoking test-statx with the runtime options
provided in the main() function of test-statx.c

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>