riscv: improve exception code naming

The existing names were inherited from arm64, but we should prefer
RISC-V terminology. Change the prefix to SCAUSE, and further change the
names to better match the RISC-V spec and be more consistent with one
another. Also, remove two codes that are not defined for S-mode (machine
and hypervisor ecall).

While here, apply style(9) to some condition checks.

Reviewed by: kp
Discussed with: jrtc27
Differential Revision: https://reviews.freebsd.org/D26918