* tm.texi (DEFAULT_RTX_COSTS): Document new macro.
        * arm.h (DEFAULT_RTX_COSTS): Define instead of RTX_COSTS.
        * cse.c (rtx_cost): Provide a default case in an enumeration
        switch, and call DEFAULT_RTX_COSTS if it's defined.

Co-Authored-By: Nick Clifton <nickc@cygnus.com>
Co-Authored-By: Richard Earnshaw <rearnsha@arm.com>

From-SVN: r18513