riscv: improve KTR_TRAP trace entries

For more informative records of exceptions, include key details such as
the exception code and stval register contents. Remove the curthread
argument as it is redundant (saved with every ktr entry), and the
trapframe as it is somewhat meaningless.

Add a new KTR_TRAP trace record for interrupts.

Reviewed by: markj, jhb
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40584