Enable the glusterfs storage driver only on 64bit architectures

Newer versions of GlusterFS support only 64bit architectures [1];
hence, restrict the usage of GlusterFS to enable the storage driver
for it only on 64bit Linux architectures.

[1] https://tracker.debian.org/news/1444468/accepted-glusterfs-110-1-source-into-experimental/

Gbp-Dch: Short