selftests/powerpc: Add test for DSCR inheritence across fork & exec

This patch adds a test case to verify that the changed DSCR value inside
any process would be inherited to it's child across the fork and exec
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>