panic_immediate_abort requires abort as a panic strategy

Guide `panic_immediate_abort` users away from `-Cpanic=unwind` and
towards `-Cpanic=abort` to avoid an accidental use of the feature with
the unwind strategy, e.g., on a targets where unwind is the default.

The `-Cpanic=unwind` combination doesn't offer the same benefits, since
the code would still be generated under the assumption that functions
implemented in Rust can unwind.

Author: tmiasko

library/core/src/panicking.rs

@@ -29,6 +29,9 @@
 use crate::fmt;
 use crate::panic::{Location, PanicInfo};
 
+#[cfg(feature = "panic_immediate_abort")]
+const _: () = assert!(cfg!(panic = "abort"), "panic_immediate_abort requires -C panic=abort");
+
 // First we define the two main entry points that all panics go through.
 // In the end both are just convenience wrappers around `panic_impl`.