powerpc/ptrace: split out ADV_DEBUG_REGS related functions.

Move ADV_DEBUG_REGS functions out of ptrace.c, into
ptrace-adv.c and ptrace-noadv.c

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Squash in fixup patch from Christophe]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e2bd7d275bd5933d848aad4fee3ca652a14d039b.1582848567.git.christophe.leroy@c-s.fr