IMPROVED ARM INTERRUPT HANDLING

```
Description: ARM interrupt handling performance could be improved in some
ways. One easy way is to use a pointer to the context save
area in g_current_regs instead of using up_copystate so much.

This approach is already implemented for the ARM Cortex-M0,
Cortex-M3, Cortex-M4, and Cortex-A5 families. But still needs
to be back-ported to the ARM7 and ARM9 (which are nearly
identical to the Cortex-A5 in this regard). The change is
*very* simple for this architecture, but not implemented.
Status: Open. But complete on all ARM platforms except ARM7 and ARM9.
Priority: Low.
```