Add builtin rust platform decls with examples (#144)

* Add builtin rust platform decls with examples

* Remove empty files

* Tidy up

* Add aliases for each OS and each CPU arch as well

* Tidy up
diff --git a/rust/platform/BUILD b/rust/platform/BUILD
new file mode 100644
index 0000000..bb34d9b
--- /dev/null
+++ b/rust/platform/BUILD
@@ -0,0 +1,5 @@
+package(default_visibility = ["//visibility:public"])
+
+load(":platform.bzl", "declare_config_settings")
+
+declare_config_settings()