Merge remote-tracking branch 'remotes/stsquad/tags/pull-6.0-rc2-fixes-060421-1' into...

Merge remote-tracking branch 'remotes/stsquad/tags/pull-6.0-rc2-fixes-060421-1' into staging

Testing updates:

  - fix x86_64 cross compilers
  - don't use registry for non-x86 containers
  - add valid host types for given cross compile containers
  - clean up i386 code16 test with explicit -no-pie
  - relax sha1.py gdbstub test
  - add more gdbstub documentation
  - remove annoying warning on gitlab
  - test dtrace backend in gitlab

# gpg: Signature made Tue 06 Apr 2021 15:55:53 BST
# gpg:                using RSA key 6685AE99E75167BCAFC8DF35FBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>" [full]
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-6.0-rc2-fixes-060421-1:
  gitlab-ci.yml: Test the dtrace backend in one of the jobs
  gitlab-ci.yml: Fix the filtering for the git submodules
  docs/system/gdb.rst: Document how to debug multicore machines
  docs/system/gdb.rst: Add some more heading structure
  tests/tcg: relax the next step precision of the gdb sha1 test
  tests/tcg/i386: force -fno-pie for test-i386
  tests/tcg/i386: expand .data sections for system tests
  tests/tcg/configure.sh: make sure we pick up x86_64 cross compilers
  tests/tcg: add concept of container_hosts
  tests/docker: don't set DOCKER_REGISTRY on non-x86_64
  tests/tcg: update the defaults for x86 compilers

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