Check alignment of `fp` in `unwind_frame`

[dg612_cam.ac.uk]

Summary

A misaligned frame pointer is certainly not a valid frame pointer and can cause panics when it is loaded from later in the code.

[jrtc27]

can cause panics when it is loaded from later in the code.

Not unless you turn on the bit for trapping on unaligned accesses. It's only a problem for downstream CHERI where you don't get unaligned capabilities.

[andrew]

Reverted as it breaks the build.

[andrew]

I think you either want __is_aligned or to define is_aligned in FreeBSD. I only see is_aligned in CheriBSD.