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

This patch adds ptrace interface test for TAR, PPR, DSCR
registers. This also adds ptrace interface based helper
functions related to 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>