build: unify Label function signatures

The Context.Label method and LABEL global function did not agree. Also
breaks the convention I'd like to set that capitalized functions must
agree with existing Go assembly syntax.

To help avoid a conflict with `avo.Label`, attributes were moved to
their own package.

Fixes #35

printer/goasm_test.go

@@ -3,7 +3,7 @@ package printer_test
 import (
 	"testing"
 
-	"github.com/mmcloughlin/avo"
+	"github.com/mmcloughlin/avo/attr"
 	"github.com/mmcloughlin/avo/build"
 	"github.com/mmcloughlin/avo/printer"
 	"github.com/mmcloughlin/avo/reg"
@@ -47,7 +47,7 @@ func TestTextDecl(t *testing.T) {
 
 	ctx.Function("withattr")
 	ctx.SignatureExpr("func()")
-	ctx.Attributes(avo.NOSPLIT | avo.TLSBSS)
+	ctx.Attributes(attr.NOSPLIT | attr.TLSBSS)
 	ctx.RET()
 
 	AssertPrintsLines(t, ctx, printer.NewGoAsm, []string{