* cpu.h: corrected the registers in Context_Control and
in CPU_Interrupt_frame to correspond to the saved frame in cpu_asm.S
Also switched on CPU_ISR_PASSES_FRAME_POINTER.
* cpu_asm.S: Moved the restore part of _CPU_Context_switch for
easier reading. Fixed _CPU_Context_restore, it now moves the
argument and branches to a label in _CPU_Context_switch. Removed
unnecessary saves of registers in context switch and irq handling.
Rewrote irq code to call the C helper. Added some documentation
* irq.c: New file derived from c4x and nios2.
7c4c284c4a