Make memtag_stack a variant

Otherwise we cannot disable memtag_stack on first stage init, because it
statically links in a lot of libraries.

This was benign, but if we want to add a stack history buffer, we do not
want to emit these instructions for first stage init.

Bug: 309446520
Change-Id: Ifbdcbe5ba06eb46970bc1e9b64a26c1ca59ba5f2