Rollup merge of #87787 - hyd-dev:c-unwind, r=RalfJung

Use `C-unwind` ABI for `__rust_start_panic` in `panic_abort`

The function originally has `C` ABI but is called using `C-unwind` ABI in `std`:
https://github.com/rust-lang/rust/blob/d4ad1cfc63ba5824196bfb2370451ddb5af2e020/library/std/src/panicking.rs#L49-L54
Which might be [problematic](https://github.com/rust-lang/miri/pull/1745#discussion_r596096876) and triggers this [Miri error](https://github.com/rust-lang/rust/issues/87778#issuecomment-893306222):
```
error: Undefined Behavior: calling a function with ABI C using caller ABI C-unwind
   --> /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:672:9
    |
672 |         __rust_start_panic(obj)
    |         ^^^^^^^^^^^^^^^^^^^^^^^ calling a function with ABI C using caller ABI C-unwind
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
```
Changing the ABI of the function to `C-unwind` fixes the error above.

library/panic_abort/src/lib.rs

@@ -15,6 +15,7 @@
 #![feature(staged_api)]
 #![feature(rustc_attrs)]
 #![feature(asm)]
+#![feature(c_unwind)]
 
 #[cfg(target_os = "android")]
 mod android;
@@ -30,7 +31,7 @@
 
 // "Leak" the payload and shim to the relevant abort on the platform in question.
 #[rustc_std_internal_symbol]
-pub unsafe extern "C" fn __rust_start_panic(_payload: *mut &mut dyn BoxMeUp) -> u32 {
+pub unsafe extern "C-unwind" fn __rust_start_panic(_payload: *mut &mut dyn BoxMeUp) -> u32 {
     // Android has the ability to attach a message as part of the abort.
     #[cfg(target_os = "android")]
     android::android_set_abort_message(_payload);