(restore_gpr): Use N-1 as subscript into sc_gpr (sc_gpr[0] => $1).
Before general regs, restore from sc_mdlo and sc_mdhi.
Don't treat sp, fp specially; use restore_gpr for them too.
For final return, store user $1 value beyond top of user stack ahead of
time; Then use $1 to hold the user PC, and restore it from the stack in the
delay slot.
Fix some SCP references in register loads to use SCPREG instead.
Load SCPREG->sc_pc into $24 and jump to it, restoring $at in the delay slot.
This still leaves $24 clobbered.