arm: Fix handling of undefined instructions in kernel mode

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

While here, fix the panic string to not include a newline.

Reviewed by: andrew
Obtained from: Stormshield
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D42970