third_party: Add support for building libffi for ARM.

third_party/libffi/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 config("libffi_config") {
   include_dirs = []
 
   if (current_cpu == "x64") {
     include_dirs += [ "include/x86_64-unknown-linux-gnu/include" ]
   }
+
+  if (current_cpu == "arm") {
+    include_dirs += [ "include/arm-unknown-linux-gnu/include" ]
+  }
 }
 
 static_library("libffi") {
   sources = [
     "src/src/closures.c",
     "src/src/java_raw_api.c",
     "src/src/prep_cif.c",
     "src/src/raw_api.c",
     "src/src/types.c",
   ]
 
   include_dirs = [ "src/include" ]
 
   if (current_cpu == "x64") {
     sources += [
       "src/src/x86/ffi.c",
       "src/src/x86/ffi64.c",
       "src/src/x86/sysv.S",
       "src/src/x86/unix64.S",
     ]
 
     include_dirs += [ "include/x86_64-unknown-linux-gnu" ]
   }
 
+  if (current_cpu == "arm") {
+    sources += [
+      "src/src/arm/ffi.c",
+      "src/src/arm/sysv.S",
+    ]
+
+    include_dirs += [ "include/arm-unknown-linux-gnu" ]
+  }
+
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
 
   public_configs = [ ":libffi_config" ]
 }