target/m68k: In dump_address_map() check for memory access failures

In dump_address_map(), use address_space_ldl() instead of ldl_phys().
This allows us to check whether the memory access failed.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20181210165636.28366-2-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>