kernel: sign GKI modules

Only signed GKI modules are permitted to export symbols listed in the
android/abi_gki_protected_exports file. Attempting to export these
symbols from an unsigned module will result in the module failing to
load, with a 'Permission denied' error message.

Change-Id: Ie15b00a6c288eda21b319eb0f735cf4f9e6e7933