alter_fp_rounding_mode Is Not Panic Safe

### Describe the bug

alter_fp_rounding_mode makes use of `fesetround` in order to alter the rounding mode on various architectures

```
let current = fegetround();
fesetround(if UPPER { FE_UPWARD } else { FE_DOWNWARD });
let result = operation(lhs, rhs);
fesetround(current);
result
```

Unfortunately if `operation` panics, `fesetround` will never revert the rounding mode. `Drop` would be a more reliable way around this, although there are still situations where `Drop` may not be called

### To Reproduce

_No response_

### Expected behavior

Ideally we wouldn't modify global thread-local state at all, but I'm not sure if this can be avoided

### Additional context

_No response_

Start by locating alter_fp_rounding_mode and tracing its fesetround usage across the supported architectures. Reproduce the panic path around operation and determine whether the rounding mode is restored afterward; done means panic handling does not leave the thread-local rounding mode altered, subject to the platform constraints described in the issue.

Written by the indexing model from the issue text.