powerpc: Move ptrace into a subdirectory.

In order to allow splitting of ptrace depending on the different
CONFIG_ options, create a subdirectory dedicated to ptrace and move
ptrace.c and ptrace32.c into it.

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/9ebcbe37834e9d447dd97f4381084795a673260c.1582848567.git.christophe.leroy@c-s.fr