powerpc/ptrace: Enable support for NT_PPC_CVMX

This patch enables support for TM checkpointed VMX register
set ELF core note NT_PPC_CVMX based ptrace requests through
PTRACE_GETREGSET, PTRACE_SETREGSET calls. This is achieved
through adding a register set REGSET_CVMX in powerpc
corresponding to the ELF core note section added. It
implements the get, set and active functions for this new
register set added.

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>