Simplify arguments to signal handlers on mips.

• Use ksi_addr directly as si_addr in the siginfo instead of the 'badvaddr' register.
• Remove a duplicate assignment of si_code.
• Use ksi_addr as the 4th argument to the old-style handler instead of 'badvaddr'.

Reviewed by: brooks, kevans
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23013