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

pass/textflag.go

@@ -2,6 +2,7 @@ package pass
 
 import (
 	"github.com/mmcloughlin/avo"
+	"github.com/mmcloughlin/avo/attr"
 )
 
 // IncludeTextFlagHeader includes textflag.h if necessary.
@@ -26,7 +27,7 @@ func IncludeTextFlagHeader(f *avo.File) error {
 // requirestextflags returns whether the file uses flags in the textflags.h header.
 func requirestextflags(f *avo.File) bool {
 	for _, s := range f.Sections {
-		var a avo.Attribute
+		var a attr.Attribute
 		switch s := s.(type) {
 		case *avo.Function:
 			a = s.Attributes