tools: Introduce $(xenlibs-ldlibs, ) macro

tools: Introduce $(xenlibs-ldlibs, ) macro

This can be used when linking against multiple in-tree Xen libraries,
and avoid duplicated flags. It can be used instead of multiple
$(LDLIBS_libxen*).

For now, replace the open-coding in libs.mk.

The macro $(xenlibs-libs, ) will be useful later when only the path to
the libraries is wanted (e.g. for checking for dependencies).

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>