ast,build: allow setting data attributes

Previously GLOBL/DATA attributes are set to RODATA. This diff removes
that default and allows them to be set through the DataAttributes
context method.

Updates #2

build/context.go

@@ -107,6 +107,10 @@ func (c *Context) StaticGlobal(name string) operand.Mem {
 	return c.global.Base()
 }
 
+func (c *Context) DataAttributes(a avo.Attribute) {
+	c.activeglobal().Attributes = a
+}
+
 func (c *Context) AddDatum(offset int, v operand.Constant) {
 	if err := c.activeglobal().AddDatum(avo.NewDatum(offset, v)); err != nil {
 		c.AddError(err)

build/global.go

@@ -24,8 +24,10 @@ func TEXT(name, signature string) {
 
 func LABEL(name string) { ctx.Label(avo.Label(name)) }
 
-func GLOBL(name string) operand.Mem {
-	return ctx.StaticGlobal(name)
+func GLOBL(name string, a avo.Attribute) operand.Mem {
+	g := ctx.StaticGlobal(name)
+	ctx.DataAttributes(a)
+	return g
 }
 
 func DATA(offset int, v operand.Constant) {

build/pseudo.go

@@ -1,6 +1,7 @@
 package build
 
 import (
+	"github.com/mmcloughlin/avo"
 	"github.com/mmcloughlin/avo/operand"
 	"github.com/mmcloughlin/avo/reg"
 
@@ -46,6 +47,7 @@ func (c *Context) Store(src reg.Register, dst gotypes.Component) {
 
 func (c *Context) ConstData(name string, v operand.Constant) operand.Mem {
 	g := c.StaticGlobal(name)
+	c.DataAttributes(avo.RODATA | avo.NOPTR)
 	c.AppendDatum(v)
 	return g
 }