Improve error message of calling unsupported non-"C"/"system"-ABI foreign function

[ghost]

Miri currently reports the following foo() call has ABI-mismatch UB:

#[cfg(unix)]
extern "Rust" { // or any non-"C" ABI
    fn foo();
}

#[cfg(windows)]
extern "C" { // or any non-"system" ABI
    fn foo();
}

fn main() {
    unsafe {
        foo();
    }
}

Output when targeting Linux (and maybe also macOS):

error: Undefined Behavior: calling a function with ABI C using caller ABI Rust
  --> src/main.rs:13:9
   |
13 |         foo();
   |         ^^^^^ calling a function with ABI C using caller ABI Rust
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavio

Output when targeting Windows:

error: Undefined Behavior: calling a function with ABI system using caller ABI C
  --> <anon>:13:9
   |
13 |         foo();
   |         ^^^^^ calling a function with ABI system using caller ABI C
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior

However, to my knowledge, that's not UB -- it's just unsupported by Miri (and Miri can't assume the function has "C" or "system" ABI since Miri doesn't know about it). I believe that is because of the overzealous check_abi() call before the long match in src/shims/{posix,windows}/foreign_items.rs. The ABI is checked to match the system one ("system" on Windows, "C" otherwise) no matter the callee is recognized as a shim or an unsupported foreign function.

Therefore, this PR removes the check_abi() call before the match and inserts a check_abi() call to each non-wildcard match.

[RalfJung]

Thanks a lot, looking good. :-)
@bors r+

[bors]

📌 Commit 7ec919d has been approved by RalfJung

[bors]

⌛ Testing commit 7ec919d with merge 80d6b56...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing 80d6b56 to master...