[MTE] do not enable memtag-heap for memtag-globals

Resolves TODO

I am not 100 % sure what the link in Phabricator contained, because
the changesets are no longer accessible. I assume it was not possible
to build with memtag-stack but not memtag-heap. Stuff still builds
and works.

Change-Id: I79a9e6bf31c4b53c7df4ec7c872429f927abc509