selftests/powerpc: Add test for DSCR value inheritence across fork

This patch adds a test to verify that the changed DSCR value inside any
process would be inherited to it's child process across the fork system
call.

Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>