selftests/powerpc: Add ptrace tests for TAR, PPR, DSCR in TM

This patch adds ptrace interface test for TAR, PPR, DSCR
registers inside TM context. This also adds ptrace
interface based helper functions related to checkpointed
TAR, PPR, DSCR register access.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>