powerpc: Call gzip with -n

The timestamps recorded in the .gz files add no value.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>