Maintainer updates for June (gitlab, semihosting, plugins, virtio-gpu)

 - mark s390x runner system tests as allow_fail
 - build semihosting once
 - add register write support to plugins
 - add virtual memory write support to plugins
 - add harder memory read/write support to plugins
 - add patcher plugin and tests
 - re-stock virtio-gpu MAINTAINERS
 - fix context init for Venus fences