arm: Fix handling of undefined instructions in kernel mode

[markj]

Summary

Only panic if no undefined instruction handler matched the exception.
This can arise in practice if the VFP is enabled lazily.

Obtained from: Stormshield
Sponsored by: Klara, Inc.
Sponsored by: Stormshield

[andrew]

undefinedinstruction needs some clean ups, e.g. lets hope we don't have a misaligned PC from the kernel or that fault_pc doesn't fault, but they are outside the scope of this change.