riscv: Tidy panic messages for exceptions

• Remove trailing newlines

• Be consistent about the format used to print pointer values

• Print the trap value for access faults (it is the faulting address if non-zero) and illegal instructions (it is the first N bytes of the decoded instruction if non-zero)

Reviewed by: markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D41786