m68k/uaccess: Revive 64-bit get_user()

Revive support for 64-bit get_user(), which was disabled in commit
d94af931

 ("[PATCH] m68k: clean up uaccess.h") due to a "broken"
typeof in (then brand new) gcc-4.1.

  - Keep on using u64 for the temporary, as __typeof__() doesn't drop
    the const qualifier,
  - Move it into a union (like mips32 does) to get rid of the cast, as
    using get_user() to fetch a __user pointer would cause a "cast to
    pointer from integer of different size" warning otherwise.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Martijn Coenen <maco@android.com>