hexdump: Add support for sandbox

The current implementation outputs an address as a pointer. Update the
code to use an address instead, respecting the 32/64 nature of the CPU.

Add some initial tests copied from print_test_display_buffer(), just the
ones that can pass with the current implementation.

Note that for this case print_hex_dump() and print_bufffer() produce the
same result. For now the tests are duplicated sine we have separate
functions.

Signed-off-by: Simon Glass <sjg@chromium.org>