usercopy: Add tests for all get_user() sizes

The existing test was only exercising native unsigned long size
get_user(). For completeness, we should check all sizes. But we
must skip some 32-bit architectures that don't implement a 64-bit
get_user().

These new tests actually uncovered a bug in ARM's 64-bit get_user()
zeroing.

Signed-off-by: Kees Cook <keescook@chromium.org>