selftests/powerpc: Add ptrace tests for VSX, VMX registers in TM

This patch adds ptrace interface test for VSX, VMX registers
inside TM context. This also adds ptrace interface based helper
functions related to chckpointed VSX, VMX registers 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>