Added instrumentation for the exit function

[gregory-swiader]

• The exit function is now intercepted and an error message is printed
• Added three lines to machine.c to ensure that it compiles with make pedantic out of the box
• The intercepts also successfully compile with make pedantic
• Kept formatting consistent with other source files

[mhkarsten]

I took a look at this code and tested it out on my machine it does indeed work as expected. I have a few small notes for improvement.

1. Because of the override, a few of the test files that use exit will print the message from your hook. An easy fix as you just have to include intercept.h and change the exit() calls to orig_exit().
2. These should be replaced in codegrade like happens with the ijvm.h i believe. Just make sure Atze (or whoever manages that) is aware.
3. Your error message could be a bit more scary :) maybe something along the lines of "ALERT! INVALID USE OF EXIT() DETECTED. You will risk a failing grade if you submit with exit() in your code." or a "you will fail any test that triggers exit() in your code". I joke a bit, but something that makes it clear they risk failure by using it.
4. More of a minor point, but make sure to add a "DO NOT MODIFY" comment like in ijvm.h