Add libc++ and libc++_static aliases for SDK builds.

c++_shared and c++_static are named libc++ and libc++_static in every
other mode. Allow the same names so targets which are built for both
host and device with SDK may specify a single name for the STL.

Test: external/boringssl is able to use "libc++_static" as the STL.
Change-Id: Ibc579c7e24ddb8de12fbfc6e1b5d80648efa589e

cc/stl.go

@@ -50,6 +50,10 @@ func (stl *stl) begin(ctx BaseModuleContext) {
 				"stlport_shared", "stlport_static",
 				"gnustl_static":
 				return "ndk_lib" + s
+			case "libc++":
+				return "ndk_libc++_shared"
+			case "libc++_static":
+				return "ndk_libc++_static"
 			case "none":
 				return ""
 			default: