Add mtectrl to the base image.

This is a tiny utility (11Kb) that implements the bootloader switch
interface for MTE:
https://source.android.com/docs/security/memory-safety/bootloader-support

It is only useful on armv8.5+ devices, but there is no separate GSI
image for those.

Bug: 242244112
Test: ls /system/bin/mtectrl
Change-Id: I2696a76d5828007a2db486f41064bcfc0e89ec36