riscv: Save gp in the trapframe in both modes

[jhb]

Summary

Similar to d95fbf4e1a12565908b04b442263fe60c9e890b4, always save gp in
the trapframe even though it is only restored when returning to user
mode. This is mostly a debugging aid so that dump_regs() doesn't
print out random stack garbage as the value of gp for kernel faults
(e.g. sysctl debug.kdb.trap=1) as well as keeping kgdb's trapframe
unwinder from reporting bogus values of $gp for lower frames.

Sponsored by: DARPA

[jrtc27]

For the title + summary, RISC-V registers don't have % or $ sigils

[jhb]

Oh, the % is spurious for assembly certainly. In gdb the registers are named '$gp' (that is, it is 'p $gp' down in a lower frame that would give you an incorrect value), so I think $gp in the context of gdb makes sense.

[jhb]

I could also spell out "global pointer" in the summary if that is better

[jrtc27]

In D41699#950348, @jhb wrote:

I could also spell out "global pointer" in the summary if that is better

The current form seems reasonable