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

This patch adds ptrace interface test for TAR, PPR, DSCR
registers inside suspended TM context.

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>