librustc_llvm: default to libc++ for darwin

src/librustc_llvm/build.rs

@@ -243,6 +243,8 @@ fn main() {
         "c++"
     } else if target.contains("freebsd") {
         "c++"
+    } else if target.contains("darwin") {
+        "c++"
     } else if target.contains("netbsd") && llvm_static_stdcpp.is_some() {
         // NetBSD uses a separate library when relocation is required
         "stdc++_pic"