ir,build: pragma support (#97)

Adds support for arbitrary compiler directives.

Fixes #15

printer/stubs_test.go

@@ -0,0 +1,28 @@
+package printer_test
+
+import (
+	"testing"
+
+	"github.com/mmcloughlin/avo/build"
+	"github.com/mmcloughlin/avo/printer"
+)
+
+func TestStubsPragmas(t *testing.T) {
+	ctx := build.NewContext()
+	ctx.Function("f")
+	ctx.Pragma("noescape")
+	ctx.Pragma("linkname f remote.f")
+	ctx.SignatureExpr("func(x *uint64)")
+	ctx.RET()
+
+	AssertPrintsLines(t, ctx, printer.NewStubs, []string{
+		"// Code generated by avo. DO NOT EDIT.",
+		"",
+		"package printer",
+		"",
+		"//go:noescape",
+		"//go:linkname f remote.f",
+		"func f(x *uint64)",
+		"",
+	})
+}