powerpc/ptrace: Enable support for NT_PPPC_TAR, NT_PPC_PPR, NT_PPC_DSCR

This patch enables support for running TAR, PPR, DSCR registers
related ELF core notes NT_PPPC_TAR, NT_PPC_PPR, NT_PPC_DSCR based
ptrace requests through PTRACE_GETREGSET, PTRACE_SETREGSET calls.
This is achieved through adding three new register sets REGSET_TAR,
REGSET_PPR, REGSET_DSCR in powerpc corresponding to the ELF core
note sections added in this regad. It implements the get, set and
active functions for all these new register sets 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>