TCG Plugins initial implementation

  - use --enable-plugins @ configure
  - low impact introspection (-plugin empty.so to measure overhead)
  - plugins cannot alter guest state
  - example plugins included in source tree (tests/plugins)
  - -d plugin to enable plugin output in logs
  - check-tcg runs extra tests when plugins enabled
  - documentation in docs/devel/plugins.rst

# gpg: Signature made Mon 28 Oct 2019 15:13:23 GMT
# 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-tcg-plugins-281019-4: (57 commits)
  travis.yml: enable linux-gcc-debug-tcg cache
  MAINTAINERS: add me for the TCG plugins code
  scripts/checkpatch.pl: don't complain about (foo, /* empty */)
  .travis.yml: add --enable-plugins tests
  include/exec: wrap cpu_ldst.h in CONFIG_TCG
  accel/stubs: reduce headers from tcg-stub
  tests/plugin: add hotpages to analyse memory access patterns
  tests/plugin: add instruction execution breakdown
  tests/plugin: add a hotblocks plugin
  tests/tcg: enable plugin testing
  tests/tcg: drop test-i386-fprem from TESTS when not SLOW
  tests/tcg: move "virtual" tests to EXTRA_TESTS
  tests/tcg: set QEMU_OPTS for all cris runs
  tests/tcg/Makefile.target: fix path to config-host.mak
  tests/plugin: add sample plugins
  linux-user: support -plugin option
  vl: support -plugin option
  plugin: add qemu_plugin_outs helper
  plugin: add qemu_plugin_insn_disas helper
  plugin: expand the plugin_init function to include an info block
  ...

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