Support load/save of floating point & vector registers.

[r4]

Referring to the non-integer registers of any platform, it would be nice to store these alongside each runnable trap frame.
We probably want to define multiple sets of registers (say one program uses vector regs, but not FPU or the opposite), that way we only need to save and restore what's used (or what's available).
We also should track the state of each set: has it been used at all (uninit), has it been used but not updated (clean), used and has modified state (dirty).