Use prebuilt NDK libc++ for rv64.

Bug: None
Test: None
Change-Id: Iebd8bce9a72a2206dfb3c6311ec9139f018df6a8

cc/stl.go

@@ -80,8 +80,7 @@ func (stl *stl) begin(ctx BaseModuleContext) {
 			return ""
 		}
 		s = deduplicateStlInput(s)
-		archHasNDKStl := ctx.Arch().ArchType != android.Riscv64
-		if ctx.useSdk() && ctx.Device() && archHasNDKStl {
+		if ctx.useSdk() && ctx.Device() {
 			switch s {
 			case "", "system":
 				return "ndk_system"