selftests/powerpc: Add ptrace tests for GPR/FPR registers in TM

This patch adds ptrace interface test for GPR/FPR registers
inside TM context. This adds ptrace interface based helper
functions related to checkpointed GPR/FPR 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>