Add target.linux for linux kernel based targets

This includes android, linux_bionic, and linux_glibc currently.

Test: m nothing
Change-Id: I088eab583eee695c1f61a65a79a302837e5f4577
diff --git a/android/arch.go b/android/arch.go
index db017fd..eaa35a4 100644
--- a/android/arch.go
+++ b/android/arch.go
@@ -468,7 +468,7 @@
 		"Android64",
 		"Android32",
 		"Bionic",
-		// TODO(dwillemsen): "Linux",
+		"Linux",
 		"Not_windows",
 		"Arm_on_x86",
 		"Arm_on_x86_64",
@@ -479,7 +479,7 @@
 		for _, archType := range osArchTypeMap[os] {
 			targets = append(targets, os.Field+"_"+archType.Name)
 
-			if false { // TODO(dwillemsen): os.Linux()
+			if os.Linux() {
 				target := "Linux_" + archType.Name
 				if !inList(target, targets) {
 					targets = append(targets, target)
@@ -696,7 +696,7 @@
 		//         key: value,
 		//     },
 		// }
-		if false { // TODO(dwillemsen): os.Linux()
+		if os.Linux() {
 			field = "Linux"
 			prefix = "target.linux"
 			a.appendProperties(ctx, genProps, targetProp, field, prefix)