Moved exception vector code out of interrupts.s and into evt.s

This code is not necessarily board specific
Renamed arch dir to mach and moved most assembly code to c code
Fixed register offsets for VIC and SIC

Redid uart0.s in C code
Removed irq_handler function in _interrupts.s
Renamed evt.s to _evt.s and renamed isr_register_handler to
irq_register_isr to stick to naming conventions