S390: Add symfile-mem

S390: Add symfile-mem

For some reason symfile-mem.o is not included in the configuration for
"s390*-*-linux*".  It was added to the configuration of most GNU/Linux
targets with a patch from Andrew Cagney:

  "Add symfile-mem to all linux targets" --
  https://sourceware.org/ml/gdb-patches/2005-02/msg00053.html

But the s390 target was overlooked at that time.  Thus the command
"add-symbol-file-from-memory" is missing and VDSO symbols are not loaded.

This is fixed.

gdb/ChangeLog:

	* configure.tgt (s390*-*-linux*): Add symfile-mem.o.