Merge remote-tracking branch 'remotes/stsquad/tags/pull-testing-tcg-plugins-270520...

Testing and one plugin fix:

  - support alternates for genisoimage to test/vm
  - add clang++ to clang tests
  - fix record/replay smoke test
  - enable more softfloat tests
  - better detection of hung gdb
  - upgrade aarch64 tcg test x-compile to gcc-10
  - fix plugin cpu_index clash vs threads

# gpg: Signature made Wed 27 May 2020 14:29:20 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-testing-tcg-plugins-270520-1:
  tests/tcg: add new threadcount test
  linux-user: properly "unrealize" vCPU object
  cpus-common: ensure auto-assigned cpu_indexes don't clash
  tests/docker: use a gcc-10 based image for arm64 tests
  tests/docker: add debian11 base image
  tests/tcg: better detect confused gdb which can't connect
  tests/fp: split and audit the conversion tests
  tests/fp: enable extf80_le_quite tests
  tests/tcg: fix invocation of the memory record/replay tests
  travis.yml: Use clang++ in the Clang tests
  tests/vm: pass --genisoimage to basevm script
  configure: add alternate binary for genisoimage

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