Allow varargs for libc::open when it is allowed by the second argument
This PR allows `libc::open` to be called using two or three arguments as defined in https://man7.org/linux/man-pages/man2/open.2.html
The presence of the third argument depends on the value of the second argument. If the second argument dictates that the third argument is *required* miri will emit an error if the argument is missing. If the second argument does *not* require a third argument, then the argument is ignored and passed as 0 internally (it would be ignored by libc anyway)
also ignore 'thread leaks' with -Zmiri-ignore-leaks
This is a step towards https://github.com/rust-lang/miri/issues/1371. The remaining hard part would be supporting checking for memory leaks when there are threads still running. For now we elegantly avoid this problem by using the same flag to control both of these checks. :)
Report an error if a `#[no_mangle]`/`#[export_name = ...]` function has the same symbol name as a built-in shim
Implements https://github.com/rust-lang/miri/pull/1776#issuecomment-821322605.
The error looks like this:
```
error: found `malloc` symbol definition that clashes with a built-in shim
--> tests/compile-fail/function_calls/exported_symbol_shim_clashing.rs:12:9
|
12 | malloc(0);
| ^^^^^^^^^ found `malloc` symbol definition that clashes with a built-in shim
|
help: the `malloc` symbol is defined here
--> tests/compile-fail/function_calls/exported_symbol_shim_clashing.rs:2:1
|
2 | / extern "C" fn malloc(_: usize) -> *mut std::ffi::c_void {
3 | | //~^ HELP the `malloc` symbol is defined here
4 | | unreachable!()
5 | | }
| |_^
= note: inside `main` at tests/compile-fail/function_calls/exported_symbol_shim_clashing.rs:12:9
```
This does not implement "better error messages than we do currently for arg/ABI mismatches" in https://github.com/rust-lang/miri/pull/1776#issuecomment-821343175 -- I failed to remove all `check_arg_count()` and `check_abi()` (they are still used in `src/shims/intrinsics.rs` and `call_dlsym()`) and they don't receive the name of the shim.
