powerpc/bpf: Enable bpf_probe_read{, str}() on powerpc again

The commit 0ebeea8c ("bpf: Restrict bpf_probe_read{, str}() only
to archs where they work") caused that bpf_probe_read{, str}() functions
were not longer available on architectures where the same logical address
might have different content in kernel and user memory mapping. These
architectures should use probe_read_{user,kernel}_str helpers.

For backward compatibility, the problematic functions are still available
on architectures where the user and kernel address spaces are not
overlapping. This is defined CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE.

At the moment, these backward compatible functions are enabled only on x86_64,
arm, and arm64. Let's do it also on powerpc that has the non overlapping
address space as well.

Fixes: 0ebeea8c

 ("bpf: Restrict bpf_probe_read{, str}() only to archs where they work")
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/lkml/20200527122844.19524-1-pmladek@suse.com