Add support for building on Darwin hosts

Add toolchain and build rules for building on Darwin.

Change-Id: I78e21f4051b2941182121b28c9ddfa24396ada41

common/module.go

@@ -16,6 +16,7 @@ package common
 
 import (
 	"path/filepath"
+	"runtime"
 
 	"github.com/google/blueprint"
 )
@@ -33,6 +34,7 @@ type androidBaseContext interface {
 	Arch() Arch
 	Host() bool
 	Device() bool
+	Darwin() bool
 	Debug() bool
 	AConfig() Config
 }
@@ -372,6 +374,10 @@ func (a *androidBaseContextImpl) Device() bool {
 	return a.arch.HostOrDevice.Device()
 }
 
+func (a *androidBaseContextImpl) Darwin() bool {
+	return a.arch.HostOrDevice.Host() && runtime.GOOS == "darwin"
+}
+
 func (a *androidBaseContextImpl) Debug() bool {
 	return a.debug
 }