Add coroutine-based SMP support with WFI

This implements cooperative multitasking for multi-hart systems using
coroutines, enabling efficient SMP emulation with significant CPU usage
reduction.
- WFI instruction callback mechanism for power management
- CPU usage optimization: ~90% reduction in idle systems
- Maximum latency: 1ms (acceptable for typical 10ms timer interrupts)

Author: jserv

Makefile

@@ -169,6 +169,7 @@ OBJS := \
 	uart.o \
 	main.o \
 	aclint.o \
+	coro.o \
 	$(OBJS_EXTRA)
 
 deps := $(OBJS:%.o=.%.o.d)