CODEOWNERS: ping @amjoseph-nixpkgs on certain trees

I don't claim ownership of anything, but I would like to be notified
of PRs opened against the following paths where I've put a lot of
work into cleanup and deduplication:

  lib/systems
  pkgs/stdenv
  pkgs/build-support/cc-wrapper
  pkgs/development/compilers/gcc

Apparently CODEOWNERS is how you tell github you want to be notified
of such things.

.github/CODEOWNERS

@@ -23,7 +23,7 @@
 
 # Libraries
 /lib                        @edolstra @infinisil
-/lib/systems                @alyssais @ericson2314 @matthewbauer
+/lib/systems                @alyssais @ericson2314 @matthewbauer @amjoseph-nixpkgs
 /lib/generators.nix         @edolstra @Profpatsch
 /lib/cli.nix                @edolstra @Profpatsch
 /lib/debug.nix              @edolstra @Profpatsch
@@ -37,10 +37,10 @@
 /pkgs/top-level/stage.nix                        @Ericson2314 @matthewbauer
 /pkgs/top-level/splice.nix                       @Ericson2314 @matthewbauer
 /pkgs/top-level/release-cross.nix                @Ericson2314 @matthewbauer
-/pkgs/stdenv/generic                             @Ericson2314 @matthewbauer
+/pkgs/stdenv/generic                             @Ericson2314 @matthewbauer @amjoseph-nixpkgs
 /pkgs/stdenv/generic/check-meta.nix              @Ericson2314 @matthewbauer @piegamesde
-/pkgs/stdenv/cross                               @Ericson2314 @matthewbauer
-/pkgs/build-support/cc-wrapper                   @Ericson2314
+/pkgs/stdenv/cross                               @Ericson2314 @matthewbauer @amjoseph-nixpkgs
+/pkgs/build-support/cc-wrapper                   @Ericson2314 @amjoseph-nixpkgs
 /pkgs/build-support/bintools-wrapper             @Ericson2314
 /pkgs/build-support/setup-hooks                  @Ericson2314
 /pkgs/build-support/setup-hooks/auto-patchelf.sh @layus
@@ -124,7 +124,7 @@
 /doc/languages-frameworks/rust.section.md @zowoq @winterqt @figsoda
 
 # C compilers
-/pkgs/development/compilers/gcc @matthewbauer
+/pkgs/development/compilers/gcc @matthewbauer @amjoseph-nixpkgs
 /pkgs/development/compilers/llvm @matthewbauer @RaitoBezarius
 
 # Compatibility stuff