Add new system-gdbinit infrastructure

gdb/ChangeLog:

        * data-directory/Makefile.in (SYSTEM_GDBINIT_SRCDIR): New
        variable.
        (VPATH): Add SYSTEM_GDBINIT_SRCDIR.
        (SYSTEM_GDBINIT_DIR, SYSTEM_GDBINIT_INSTALL_DIR)
        (SYSTEM_GDBINIT_FILES): New variables.
        (all): Add stamp-system-gdbinit.
        (stamp-system-gdbinit): New rule.
        (clean-system-gdbinit, install-system-gdbinit)
        (uninstall-system-gdbinit): New rules.  Make them .PHONY.
        (install-only): Add dependency on install-system-gdbinit.
        (uninstall): Add dependency on uninstall-system-gdbinit.
        (clean): Add dependency on clean-system-gdbinit.
        * system-gdbinit/elinos.py: New file.
        * system-gdbinit/wrs-linux.py: New file.