libtool: s390x is also a 64bit glibc/ELF platform.

* m4/libtool.m4 (_LT_SYS_DYNAMIC_LIB): Add s390x to glibc/ELF
compatible CPUs that have /lib64 and /usr/lib64 in
sys_lib_dlsearch_path_spec.
Reported by Pavel Raiskup

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