enable LFS on GNU/Hurd

enable LFS on GNU/Hurd

at the moment, perl is compiled on GNU/Hurd without large file support.
This, other than the lacking of support for files > 2 GB, causes the
failure in the t/op/stat.t test.
The failure in stat.t happens because of the following: when filtering
devices in /dev/* using grep {} with -b/-c, stat fails for large
devices such as /dev/hdX blocks, or /dev/zero chars (this latter case
should be an Hurd-specific issue, but shouldn't matter for this case),
hence the counts done with grep {} and the ones done filtering the
output of `ls' don't match.

The attached patch enables the LFS using the same way used on linux.sh;
about t/op/lfs.t, it gets skipped at the "kernel/fs not configured to
use large files?" check, although it correctly detects sparse files and
(at least by looking at the disk usage) allocates the 5GB (or so) file
created with system() in that test.