Auto merge of #97924 - cuviper:unguarded-poison, r=Mark-Simulacrum

Avoid `thread::panicking()` in non-poisoning methods of `Mutex` and `RwLock`

`Mutex::lock()` and `RwLock::write()` are poison-guarded against panics,
in that they set the poison flag if a panic occurs while they're locked.
But if we're already in a panic (`thread::panicking()`), they leave the
poison flag alone.

That check is a bit of a waste for methods that never set the poison
flag though, namely `get_mut()`, `into_inner()`, and `RwLock::read()`.
These use-cases are now split to avoid that unnecessary call.

Author: bors