ANDROID: modpost: generate protected exports

These are symbols that unsigned modules may not export.

* kernel/module/internal.h - declares protected exports
* modpost generates protected-exports.c - defines protected exports

Bug: 393366754
Change-Id: I3e6d630542875e35754126e64bc279313fc131b9
Signed-off-by: Sid Nayyar <sidnayyar@google.com>