exec/memory_ldst: Use correct type sizes

Use uint8_t for (unsigned) byte, and uint16_t for (unsigned)
16-bit word.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210518183655.1711377-4-philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>