Kbuild: use normal compression settings for tar*-pkg

For large kernel configurations (like a distribution kernel)
targz-pkg takes a quite long time to just do the compression.
I clocked it at 15+mins for a SUSE kernel like config on a fast
system.  And tarxz and bzip2 are even slower.

The main reason is that the script that is doing the taring sets
the highest compression level (-9).  When I change it to just
use the defaults the gzip time for the same kernel goes down
to ~3 mins. I haven't tested xz and bzip, but I expect those
to be much faster too.

I'm not willing to wait that long for a small compression
gain. So just change the script to use the defaults.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>