avr: add interrupt support

Interrupts are supported using a special //go:interrupt pragma.
For example:

//go:interrupt INT0_vect
func handleINT0() {
    // do something here
}

compiler/compiler.go

@@ -1135,6 +1135,9 @@ func (c *Compiler) parseFunc(frame *Frame) error {
 	if !frame.fn.IsExported() {
 		frame.fn.LLVMFn.SetLinkage(llvm.InternalLinkage)
 	}
+	if frame.fn.IsInterrupt() && strings.HasPrefix(c.Triple, "avr") {
+		frame.fn.LLVMFn.SetFunctionCallConv(85) // CallingConv::AVR_SIGNAL
+	}
 
 	if c.Debug {
 		pos := c.ir.Program.Fset.Position(frame.fn.Pos())