Merge #661 #663

661: Add support for choosing other builtin GCC calling conventions r=philberty a=philberty

Add support for some other ABI options.

I was able to test this with linking against some C code but
these options like stdcall seem to be limited to 32 bit mode

```rust
extern "stdcall" {
    pub fn test(a: i32) -> i32;
}

extern "C" {
    fn printf(s: *const i8, ...);
}

fn main() -> i32 {
    unsafe {
        let a = 3;
        let res = test(a);

        let a = "%i\n\0";
        let b = a as *const str;
        let c = b as *const i8;

        printf(c, res);
    }
    0
}
```

```c
__attribute__ ((stdcall)) int test(int x)  {
  return x + 3;
}

```

Compiling like this:

```
$ gccrs -g -O0 -m32 -c test.rs -o test.o
$ gcc -g -O0 -m32 -c lib.c -o lib.o
$ gcc -m32 -o test test.o lib.o
```

More testing will be required over time here but this was
kind of fun to see that it worked.

663: Move module output test case from compile/ to execute/ r=philberty a=CohenArthur

Fixes [this issue](https://github.com/Rust-GCC/gccrs/pull/639#discussion_r705922519) by `@tschwinge` 

I think this shows that we need to figure out a better way to keep test files and their modules, as pointed out by `@dkm` [here](https://github.com/Rust-GCC/gccrs/pull/639#issuecomment-905457557). Here, the modules/ directory is a straight up duplicate of the other one in compile/ which I'm not a fan of.

I have no experience with having a separate .exp in the subdir but I can try and I think that it would be better. Maybe it can also be simpler if the regex to match *.rs files only looks inside one level of subdirectories? But I can think of a few places where this might be an issue

Co-authored-by: Philip Herron <philip.herron@embecosm.com>
Co-authored-by: CohenArthur <arthur.cohen@epita.fr>