tools/testing/cxl: require 64-bit

tools/testing/cxl: require 64-bit

size_t is limited to 32-bits and so the gen_pool_alloc() using
the size of SZ_64G would map to 0, triggering a low allocation
which is not expected. Force the dependency on 64-bit for cxl_test
as that is what it was designed for.

This issue was found by build test reports when converting this
driver as a proper upstream driver.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20221219195050.325959-1-mcgrof@kernel.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>