Merge remote-tracking branch 'remotes/huth-gitlab/tags/pull-request-2020-08-05' into staging

* Test rx-softmmu, avr-softmmu, Centos7 and Debian on gitlab-CI
* Fix compiler warning on 32-bit big endian systems
* Remove remainders of libqemustub.a

# gpg: Signature made Wed 05 Aug 2020 10:51:32 BST
# gpg:                using RSA key 27B88847EEE0250118F3EAB92ED9D774FE702DB5
# gpg:                issuer "thuth@redhat.com"
# gpg: Good signature from "Thomas Huth <th.huth@gmx.de>" [full]
# gpg:                 aka "Thomas Huth <thuth@redhat.com>" [full]
# gpg:                 aka "Thomas Huth <huth@tuxfamily.org>" [full]
# gpg:                 aka "Thomas Huth <th.huth@posteo.de>" [unknown]
# Primary key fingerprint: 27B8 8847 EEE0 2501 18F3  EAB9 2ED9 D774 FE70 2DB5

* remotes/huth-gitlab/tags/pull-request-2020-08-05:
  Get rid of the libqemustub.a remainders
  target/riscv/vector_helper: Fix build on 32-bit big endian hosts
  gitlab-ci: Fix Avocado cache usage
  gitlab-ci.yml: Add build-system-debian and build-system-centos jobs
  tests/acceptance: Disable the rx sash and arm cubieboard replay test on Gitlab
  tests/docker: Add python3-venv and netcat to the debian-amd64 container

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>