Add GN build targets for the rest of mojo/public

mojo/mojo_public.gypi

Index: mojo/mojo_public.gypi
diff --git a/mojo/mojo_public.gypi b/mojo/mojo_public.gypi
index a50027bd87a2560afedfc8d074ca926b7f650b69..82fb1ab8c988d818e494168d3346470b21fa5d9c 100644
--- a/mojo/mojo_public.gypi
+++ b/mojo/mojo_public.gypi
@@ -5,6 +5,7 @@
 {
   'targets': [
     {
+      # GN version: //mojo/public/c/system
       'target_name': 'mojo_system',
       'type': 'static_library',
       'defines': [
@@ -112,6 +113,7 @@
       ],
     },
     {
+      # GN version: //mojo/public/cpp/environment:standalone
       'target_name': 'mojo_environment_standalone',
       'type': 'static_library',
       'sources': [
@@ -131,6 +133,7 @@
       ],
     },
     {
+      # GN version: //mojo/public/cpp/utility
       'target_name': 'mojo_utility',
       'type': 'static_library',
       'sources': [
@@ -176,6 +179,7 @@
       ],
     },
     {
+      # GN version: //mojo/public/cpp/application
       'target_name': 'mojo_application',
       'type': 'static_library',
       'sources': [