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

attr/attr_test.go

@@ -0,0 +1,43 @@
+package attr
+
+import "testing"
+
+func TestAttributeAsm(t *testing.T) {
+	cases := []struct {
+		Attribute Attribute
+		Expect    string
+	}{
+		{0, "0"},
+		{32768, "32768"},
+		{1, "NOPROF"},
+		{DUPOK, "DUPOK"},
+		{RODATA | NOSPLIT, "NOSPLIT|RODATA"},
+		{WRAPPER | 16384 | NOPTR, "NOPTR|WRAPPER|16384"},
+		{NEEDCTXT + NOFRAME + TLSBSS, "NEEDCTXT|TLSBSS|NOFRAME"},
+		{REFLECTMETHOD, "1024"}, // REFLECTMETHOD special case due to https://golang.org/issue/29487
+	}
+	for _, c := range cases {
+		got := c.Attribute.Asm()
+		if got != c.Expect {
+			t.Errorf("Attribute(%d).Asm() = %#v; expect %#v", c.Attribute, got, c.Expect)
+		}
+	}
+}
+
+func TestAttributeContainsTextFlags(t *testing.T) {
+	cases := []struct {
+		Attribute Attribute
+		Expect    bool
+	}{
+		{0, false},
+		{32768, false},
+		{1, true},
+		{DUPOK, true},
+		{WRAPPER | 16384 | NOPTR, true},
+	}
+	for _, c := range cases {
+		if c.Attribute.ContainsTextFlags() != c.Expect {
+			t.Errorf("%s: ContainsTextFlags() expected %#v", c.Attribute.Asm(), c.Expect)
+		}
+	}
+}