MIPS: Fix build error if __xchg() is not getting inlined.

If __xchg() is not getting inlined the outline version of the function
will have a reference to __xchg_called_with_bad_pointer() which does not
exist remaining.  Fixed by using BUILD_BUG_ON() to check for allowable
operand sizes.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/705/