Auto merge of #1745 - hyd-dev:unsup-foreign-calls-are-not-ub, r=RalfJung

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

Miri currently reports the following `foo()` call has ABI-mismatch UB:
```rust
#[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](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=72afc3bd4d9fdab962422cfc2c5a2166) (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.

Author: bors