Sort file systems in CHPL_AUX_FILESYS variable

We currently use ``printchplenv --make`` to determine the path for the gen directory. Since we were not sorting the names in the environment variable for ``CHPL_AUX_FILESYS`` we wound up viewing ``CHPL_AUX_FILESYS="hdfs curl"`` as different from ``CHPL_AUX_FILESYS="curl hdfs"``. 

This fixes this problem by first sorting the string before adding the underscores for the path.