No warning when CloudABI entrypoint is missing

[mcandre]

Porting applications from libc to cloudlibc is hard work, but one problem is particularly pernicious: cloudlibc toolchains do not warn the programmer when he or she forgets basic steps for correctly wiring up the new entrypoint.

• C: No warning is presented at compilation time when the entrypoint continues to use a traditional libc-style <void|int> main([int argc[, char **argv]]) signature, as opposed to the recommended void program_main(const argdata_t *ad) signature.
• Rust: No warning is presented at compilation time when the entrypoint function does not query argdata::env::argdata().

When this happens, the user observes the resulting CloudABI executable emit no output. It would be better for the runtime to present a dummy warning that the entrypoint was misconfigured. Best of all would be to detect this at compile time, and refuse to produce a binary with the wrong entrypoint.

[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