switch to libstdc++ for standalone Android builds.

This makes it possible to target NDK API 18 (K) again.

Change-Id: Id3d1f19b2904792b4001d2ea0942cc1ab6cf732e
Reviewed-on: https://skia-review.googlesource.com/6081
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

gn/BUILD.gn

@@ -158,8 +158,8 @@ config("default") {
     ]
     cflags_cc += [
       "-isystem$ndk/sources/android/support/include",
-      "-isystem$ndk/sources/cxx-stl/llvm-libc++/libcxx/include",  # Through r12b.
-      "-isystem$ndk/sources/cxx-stl/llvm-libc++/include",  # Since r13.
+      "-isystem$ndk/sources/cxx-stl/gnu-libstdc++/4.9/include",
+      "-isystem$ndk/sources/cxx-stl/gnu-libstdc++/4.9/libs/$ndk_stdlib/include",
     ]
     ldflags += [
       "--sysroot=$ndk/platforms/$ndk_platform",
@@ -167,7 +167,7 @@ config("default") {
       "-B$ndk/toolchains/$ndk_gccdir-4.9/prebuilt/$ndk_host/$ndk_target/bin",
     ]
     lib_dirs = [
-      "$ndk/sources/cxx-stl/llvm-libc++/libs/$ndk_stdlib",
+      "$ndk/sources/cxx-stl/gnu-libstdc++/4.9/libs/$ndk_stdlib",
       "$ndk/toolchains/$ndk_gccdir-4.9/prebuilt/$ndk_host/lib/gcc/$ndk_target/4.9.x",
     ]
 
@@ -179,15 +179,7 @@ config("default") {
       ldflags += [ "-B$ndk/platforms/$ndk_platform/usr/lib64" ]
     }
 
-    libs += [
-      # Order matters here!  Keep these three in exactly this order.
-      "c++_static",
-      "c++abi",
-      "android_support",
-    ]
-    if (target_cpu == "arm") {
-      libs += [ "unwind" ]
-    }
+    libs += [ "gnustl_static" ]
   }
 
   if (is_ios) {

gn/BUILDCONFIG.gn

@@ -11,7 +11,11 @@ declare_args() {
   is_official_build = false
   is_component_build = false
   ndk = ""
-  ndk_api = 21
+  if (target_cpu == "x86" || target_cpu == "mipsel" || target_cpu == "arm") {
+    ndk_api = 18
+  } else {
+    ndk_api = 21
+  }
   sanitize = ""
 }
 declare_args() {