Add check-tcg machinary

This restores the ability to run TCG smoke tests by using our docker
infrastructure to support cross building simple tests. It represents
the first step to making better cross-architecture testing available
straight from the source tree ;-)

v2
  - fix quoting of target_compiler
  - make docker.py Py3 safe
  - tweak .travis.yml recipe
  - don't probe docker when HAVE_USER_DOCKER not set

# gpg: Signature made Thu 21 Jun 2018 07:23:45 BST
# gpg:                using RSA key FBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>"
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-tcg-testing-revivial-210618-2: (57 commits)
  .travis.yml: add check-tcg test
  tests/docker/Makefile.include: only force SID to NOCACHE if old
  docker: docker.py adding age check command
  tests/Makefile: call sub-makes with SKIP_DOCKER_BUILD=1
  docker: docker.py add check sub-command
  docker: docker.py don't conflate checksums for extra_files
  docker: docker.py use "version" to probe usage
  tests: add top-level make dependency for docker builds
  tests/tcg/i386: extend timeout for runcom test
  tests/tcg: override runners for broken tests
  tests/tcg: add run, diff, and skip helper macros
  tests/Makefile.include: add [build|clean|check]-tcg targets
  Makefile.target: add (clean-/build-)guest-tests targets
  tests/tcg/Makefile: update to be called from Makefile.target
  tests/tcg: enable building for PowerPC
  docker: move debian-powerpc-cross to sid based build
  tests/tcg: enable building for RISCV64
  tests/tcg: enable building for mips64
  tests/tcg: enable building for sparc64
  tests/tcg: enable building for sh4
  ...

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