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/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
 }