mn10300: fix kernel build failures when using gcc-4.x

Fix some build failures when using gcc-4.x for MN10300.

Firstly, __get_user() fails to build because the pointer points to a const and
__gu_val ends up being read-only:

In file included from include/linux/mempolicy.h:62,
                 from init/main.c:50:
include/linux/pagemap.h: In function 'fault_in_pages_readable':
include/linux/pagemap.h:394: error: read-only variable '__gu_val' used as 'asm' output
include/linux/pagemap.h:394: error: read-only variable '__gu_val' used as 'asm' output
include/linux/pagemap.h:394: error: read-only variable '__gu_val' used as 'asm' output
include/linux/pagemap.h:400: error: read-only variable '__gu_val' used as 'asm' output
include/linux/pagemap.h:400: error: read-only variable '__gu_val' used as 'asm' output
include/linux/pagemap.h:400: error: read-only variable '__gu_val' used as 'asm' output
make[1]: *** [init/main.o] Error 1

Secondly, gcc-4 doesn't allow casts of lvalues:

  UPD     include/linux/compile.h
...