exec.c: Use stn_p() and ldn_p() instead of explicit switches

exec.c: Use stn_p() and ldn_p() instead of explicit switches

Now we have stn_p() and ldn_p() we can use them in various
functions in exec.c that used to have their own switch-on-size code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180611171007.4165-4-peter.maydell@linaro.org