riscv: improve KTR_TRAP trace entries

[mhorne]

Summary

For more informative records of exceptions, include key details such as
the exception code and stval contents. Add a new KTR_TRAP trace record
for interrupts.

[markj]

sys/riscv/riscv/intr_machdep.c

167

KTR entries always contain a pointer to curthread (used to implement ktrdump -H), so including it here is redundant.

I wonder if the pointer to frame is ever useful?

[mhorne]

sys/riscv/riscv/intr_machdep.c

167

Good point on curthread. For frame, probably not. It has a short lifetime and it's not like we can do much with it in ddb.

[mhorne]

Drop curthread and trapframe arguments.