MIPS: Refactor 'clear_page' and 'copy_page' functions.

    Remove usage of the '__attribute__((alias("...")))' hack that aliased
    to integer arrays containing micro-assembled instructions. This hack
    breaks when building a microMIPS kernel. It also makes the code much
    easier to understand.

    [ralf@linux-mips.org: Added back export of the clear_page and copy_page
    symbols so certain modules will work again.  Also fixed build with
    CONFIG_SIBYTE_DMA_PAGEOPS enabled.]