Add port done awhile ago for the ARC cpu.

        * arc/arc.h: New file.
        * arc/arc.c: New file.
        * arc/arc.md: New file.
        * arc/initfini.c: New file.
        * arc/lib1funcs.asm: New file.
        * arc/t-arc: New file.
        * arc/xm-arc.h: New file.
        * ginclude/va-arc.h: New file.
        * ginclude/stdarg.h: Include va-arc.h ifdef __arc__.
        * ginclude/varargs.h: Likewise.
        * Makefile.in (USER_H): Add va-arc.h.
        * configure.in (arc-*-elf*): Recognize.
        * longlong.h: Add ARC support.
Mostly so I can test changes in snapshot scripts.

        * expr.c (clear_storage): Use CONST0_RTX instead of const0_rtx.
        when clearing non-BLKmode data.
Fixes sparc problem.

From-SVN: r15186