Fix GenC tail rec elimination for unit functions when body is not in Block

[zhekai-jiang]

This is to fix a newly-discovered subtle bug, following #1626. Essentially for Unit functions without a return statement, we add a return statement at the end so that it is not rewritten into an infinite while loop. We assumed before that all functions have body wrapped in a Block object, but actually this may not be the case.

(Interestingly, we didn’t change any test case, and it was not an issue on our x86/64 laptops and on CI (function bodies were always wrapped in Blocks). But I accidentally got an issue when I was testing things on a different Mac with Apple Silicon (ARM architecture), where sometimes if the function has, say, only an If statement, the body will simply be that If object instead of that If wrapped inside a Block object.)