powerpc/ptrace: create ppc_gethwdinfo()

Create ippc_gethwdinfo() to handle PPC_PTRACE_GETHWDBGINFO and
reduce ifdef mess

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/82fefcc1ec75b96cece792878217a5d85ecda0c2.1582848567.git.christophe.leroy@c-s.fr