libtool: for 64bit GNU arches, add /lib64 and /usr/lib64 to sys_lib_dlsearch_path.

* m4/libtool.m4 (_LT_SYS_DYNAMIC_LINKER) <linux*, k*bsd*-gnu>
<kopensolaris*-gnu, gnu*>: If $host_cpu contains "64", add /lib64
and /usr/lib64 to sys_lib_dlsearch_path_spec.
Reported by Orion Poplawski, Christian Rössel, Olly Betts et. al.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>