build: When targeting Android with Clang, use llvm-ar instead of NDK ar.

Without this, archives will be missing symbol tables when LTO is
enabled, which can lead to link errors.

Bug: 469376
Change-Id: I14ac52cd692d73d4773005071881463b2c60d23f
Reviewed-on: https://chromium-review.googlesource.com/719351
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#508851}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 508d46cc2897daa914fe9454000d8a8bc3eddbd2

toolchain/android/BUILD.gn

@@ -56,11 +56,12 @@ template("android_gcc_toolchain") {
       _prefix = rebase_path("$clang_base_path/bin", root_build_dir)
       cc = "$_prefix/clang"
       cxx = "$_prefix/clang++"
+      ar = "$_prefix/llvm-ar"
     } else {
       cc = "${_tool_prefix}gcc"
       cxx = "${_tool_prefix}g++"
+      ar = _tool_prefix + "ar"
     }
-    ar = _tool_prefix + "ar"
     ld = cxx
     readelf = _tool_prefix + "readelf"
     nm = _tool_prefix + "nm"