powerpc: More stack randomisation for 64bit binaries

At the moment we randomise the stack by 8MB on 32bit and 64bit tasks. Since we
have a lot more address space to play with on 64bit, lets do what x86 does and
increase that randomisation to 1GB:

before:
# for i in seq `1 10` ; do sleep 1 & cat /proc/${!}/maps | grep stack; done
fffffebc000-fffffed1000 rw-p ffffffeb000 00:00 0       [stack]
ffffff5a000-ffffff6f000 rw-p ffffffeb000 00:00 0       [stack]
fffffdb2000-fffffdc7000 rw-p ffffffeb000 00:00 0       [stack]
fffffd3e000-fffffd53000 rw-p ffffffeb000 00:00 0       [stack]
fffffad9000-fffffaee000 rw-p ffffffeb000 00:00 0       [stack]

after:
# for i in seq `1 10` ; do sleep 1 & cat /proc/${!}/maps | grep stack; done
ffff5c27000-ffff5c3c000 rw-p ffffffeb000 00:00 0       [stack]
fffebe5e000-fffebe73000 rw-p ffffffeb000 00:00 0       [stack]
fffcb298000-fffcb2ad000 rw-p ffffffeb000 00:00 0       [stack]
fffc719d000-fffc71b2000 rw-p ffffffeb000 00:00 0       [stack]
fffe01af000-fffe01c4000 rw-p ffffffeb000 00:00 0       [stack]

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