No warning when CloudABI entrypoint is missing about cloudlibc

[EdSchouten]

I agree with this. Just to give some background: when it was just C that we supported, I wanted to remove support for main() entirely at some point. Unfortunately, there are an insane number of build systems out there that do compile-based checks that depend on main() working. GNU Autoconf checks for the existence of a function by compiling something like this:

void function_you_are_looking_for(); 
void main() { function_you_are_looking_for(); }

I guess that for the existing toolchain setup that we have, we should keep main() working, even if people shouldn't make use of it.

I am currently experimenting with building CloudABI software through Bazel. With Bazel it is far less common (or even impossible?) to do compile-based feature checking, meaning that the necessity for supporting main() is likely a lot less. I could likely omit this source file entirely, meaning we simply get a hard linker error in case program_main() is missing:

https://github.com/NuxiNL/cloudlibc/blob/master/src/libc/program/program_main.c