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

This patch adds ptrace interface test for VSX, VMX registers.
This also adds ptrace interface based helper functions related
to VSX, VMX registers access. This also adds some assembly
helper functions related to VSX and VMX registers.

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>