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

build/global.go

@@ -4,13 +4,12 @@ import (
 	"flag"
 	"os"
 
+	"github.com/mmcloughlin/avo/attr"
 	"github.com/mmcloughlin/avo/buildtags"
 	"github.com/mmcloughlin/avo/gotypes"
 	"github.com/mmcloughlin/avo/operand"
 
 	"github.com/mmcloughlin/avo/reg"
-
-	"github.com/mmcloughlin/avo"
 )
 
 // ctx provides a global build context.
@@ -22,11 +21,8 @@ func TEXT(name, signature string) {
 	ctx.SignatureExpr(signature)
 }
 
-// LABEL adds a label to the active function.
-func LABEL(name string) { ctx.Label(avo.Label(name)) }
-
 // GLOBL declares a new static global data section with the given attributes.
-func GLOBL(name string, a avo.Attribute) operand.Mem {
+func GLOBL(name string, a attr.Attribute) operand.Mem {
 	// TODO(mbm): should this be static?
 	g := ctx.StaticGlobal(name)
 	ctx.DataAttributes(a)
@@ -119,11 +115,14 @@ func Store(src reg.Register, dst gotypes.Component) { ctx.Store(src, dst) }
 func Doc(lines ...string) { ctx.Doc(lines...) }
 
 // Attributes sets function attributes for the currently active function.
-func Attributes(a avo.Attribute) { ctx.Attributes(a) }
+func Attributes(a attr.Attribute) { ctx.Attributes(a) }
 
 // AllocLocal allocates size bytes in the stack of the currently active function.
 // Returns a reference to the base pointer for the newly allocated region.
 func AllocLocal(size int) operand.Mem { return ctx.AllocLocal(size) }
 
+// Label adds a label to the active function.
+func Label(name string) { ctx.Label(name) }
+
 // ConstData builds a static data section containing just the given constant.
 func ConstData(name string, v operand.Constant) operand.Mem { return ctx.ConstData(name, v) }