gunzip: cache-align write buffer memory

When using gzwrite to eMMC on an i.MX6Q board, the following warning
occurs repeatedly:
CACHE: Misaligned operation at range [4fd63318, 4fe63318]

This patch cache-aligns the memory allocation for the gzwrite writebuf,
therefore avoiding the misaligned dcache flush and the warning from
check_cache_range.

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Reviewed-by: Eric Nelson <eric@nelint.com>
Reviewed-by: Stefan Agner <stefan.agner@toradex.com>