Migrate libstdc++ to ndk_library.

Test: make checkbuild
Bug: http://b/30465923
Change-Id: I3b75b4e7fa652c83846c5978f9908e8b32d43aa7
diff --git a/libc/Android.bp b/libc/Android.bp
index c9b2f93..c67e23f 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -2110,4 +2110,10 @@
     first_version: "9",
 }
 
+ndk_library {
+    name: "libstdc++.ndk",
+    symbol_file: "libstdc++.map.txt",
+    first_version: "9",
+}
+
 subdirs = ["malloc_debug"]