build: Pass -nostdlib++ even on Android.

After https://crrev.com/c/1756721 we stopped passing -nostdlib on Android,
so the -nostdlib++ flag is not redundant and is in fact required in some
configurations to prevent linking against the NDK libc++.

Bug: 996172
Change-Id: Iabede8e5cafad29debf6b5f098a247f399cb460c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762346
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689047}

build/config/c++/BUILD.gn

@@ -94,12 +94,7 @@ config("runtime_library") {
 
     # Make sure we don't link against the system libstdc++ or libc++.
     if (is_clang) {
-      # //build/config/android:runtime_library adds -nostdlib, which suppresses
-      # linking against all system libraries.  -nostdlib++ would be redundant,
-      # and would generate an unused warning in this case.
-      if (!is_android) {
-        ldflags += [ "-nostdlib++" ]
-      }
+      ldflags += [ "-nostdlib++" ]
     } else {
       # Gcc has a built-in abs() definition with default visibility.
       # If it was not disabled, it would conflict with libc++'s abs()