Use toolchain defined in starboard path (#3763)

This prevents the duplication of native_target toolchain if it's defined
in both path android/arm/toolchain and $starboard_path/toolchain

b/291130824

starboard/loader_app/BUILD.gn

@@ -74,7 +74,7 @@ if (sb_is_evergreen_compatible && sb_evergreen_compatible_package &&
     install_content = true
     sources = [ "$root_out_dir/native_target/crashpad_handler" ]
     outputs = [ "$root_out_dir/libcrashpad_handler.so" ]
-    deps = [ "//third_party/crashpad/crashpad/handler:crashpad_handler(//starboard/android/arm/toolchain:native_target)" ]
+    deps = [ "//third_party/crashpad/crashpad/handler:crashpad_handler(//$starboard_path/toolchain:native_target)" ]
   }
 }