Remove unnecessary _stack_start from Rust code

[matthiasgoergens]

Just like __global_pointer or _start, we don't need to mention _stack_start in Rust to make use of it in the assembly.

[lispc]

i am not sure whether it is a good idea. Will the extern C code block make IDE happier?

it can make readers easy to understand? "Oh these will be a extern var named _stack_start". Many programmers knows extern C, while few knows linker stuff.

[matthiasgoergens]

Well, as mentioned in the PR description, there's a bunch of other symbols we only take from the linker, too. So _stack_start ain't special.

We should either define all of them here redundantly, or none.

(In general, this file should only be edited by people who have looked at least a bit into the linker scripts.)

[lispc]

i prefer putting them all in extern C

[matthiasgoergens]

i prefer putting them all in extern C

OK, I just did some experiments. For example, if you mistype the name of the variable in the extern "C" declaration like this:

extern "C" {
    // The address of this variable is the start of the stack (growing downwards).
    static _stack_start_this_is_a_typo: u8;
}

then you still don't get an error message. So it seems like having this declaration is no better than just adding a comment. And perhaps worse, because everyone knows that human readable comments are only there for humans and that the compiler doesn't check them; but the extern "C" variable declaration looks like it might be checked, but ain't really.

So I suggest I'll add a comment to the assembly code instead. See the new version, please.