commit missing file

nix/gdb.nix

@@ -0,0 +1,13 @@
+# TODO: gdb 8.2.1 from NixOS >= 19.XX is multiarch by default.
+# remove the following as `gdb` is already in scope
+
+{ stdenv, gdb }:
+
+gdb.override {
+  stdenv = stdenv.override {
+    targetPlatform = {
+      config = "arm-none-eabihf";
+      libc = "newlib";
+    };
+  };
+}