selftests/powerpc: Add thread based stress test for DSCR sysfs interfaces

This patch adds a test to update the system wide DSCR value repeatedly
and then verifies that any thread on any given CPU on the system must be
able to see the same DSCR value whether its is being read through the
problem state based SPR or the privilege state based SPR.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>