ir,build: pragma support (#97)

Adds support for arbitrary compiler directives.

Fixes #15

examples/pragma/pragma_test.go

@@ -0,0 +1,16 @@
+package pragma
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+//go:generate go run asm.go -out pragma.s -stubs stub.go
+
+func TestAdd(t *testing.T) {
+	got := func(x, y uint64) (z uint64) { Add(&z, &x, &y); return }
+	expect := func(x, y uint64) uint64 { return x + y }
+	if err := quick.CheckEqual(got, expect, nil); err != nil {
+		t.Fatal(err)
+	}
+}