powerpc: Increase stack gap on 64bit binaries

On 64bit there is a possibility our stack and mmap randomisation will put
the two close enough such that we can't expand our stack to match the ulimit
specified.

To avoid this, start the upper mmap address at 1GB + 128MB below the top of our
address space, so in the worst case we end up with the same ~128MB hole as in
32bit. This works because we randomise the stack over a 1GB range.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>