third_party: Add support for building libffi for ARM.

third_party/libffi/BUILD.gn

Index: third_party/libffi/BUILD.gn
diff --git a/third_party/libffi/BUILD.gn b/third_party/libffi/BUILD.gn
index 791e61be3181bb577873e31ef6fe18078d471e22..32f5ff3639666278b3197da9caa4ea109281551b 100644
--- a/third_party/libffi/BUILD.gn
+++ b/third_party/libffi/BUILD.gn
@@ -8,6 +8,10 @@ config("libffi_config") {
   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") {
@@ -32,6 +36,15 @@ static_library("libffi") {
     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" ]