all: add compiler support for interrupts

This commit lets the compiler know about interrupts and allows
optimizations to be performed based on that: interrupts are eliminated
when they appear to be unused in a program. This is done with a new
pseudo-call (runtime/interrupt.New) that is treated specially by the
compiler.

transform/interrupt_test.go

@@ -0,0 +1,24 @@
+package transform
+
+import (
+	"testing"
+
+	"tinygo.org/x/go-llvm"
+)
+
+func TestInterruptLowering(t *testing.T) {
+	t.Parallel()
+	for _, subtest := range []string{"avr", "cortexm"} {
+		t.Run(subtest, func(t *testing.T) {
+			testTransform(t, "testdata/interrupt-"+subtest, func(mod llvm.Module) {
+				errs := LowerInterrupts(mod)
+				if len(errs) != 0 {
+					t.Fail()
+					for _, err := range errs {
+						t.Error(err)
+					}
+				}
+			})
+		})
+	}
+}