stubdom: xenlibs linkfarm, ignore non-regular files

stubdom: xenlibs linkfarm, ignore non-regular files

When we will convert tools/ build system, their will be a need to
replace some use of "vpath". This will done making symbolic links.
Those symlinks are not wanted by stubdom build system when making a
linkfarm for the Xen libraries. To avoid them, we will use `find`
instead of plain shell globbing.

For example, there will be a link to "xen/lib/x86/cpuid.o" in
"tools/libs/guest/".

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>