riscv/exception.S: save the thread pointer in both modes

[christos]

Summary

The contents of frame->tf_tp are uninitialized if accessed by DTrace (in
probe context), resulting in a panic when trying to access the memory pointed to by
tp. This saves the thread pointer to the trap frame when handling
both userland and kernel exceptions.

[mhorne]

Nice!

[jhb]

sys/riscv/riscv/exception.S

46

Could we move 'sd' here so it's not duplicated?

[christos]

sys/riscv/riscv/exception.S

46

D39894 fixes this.