gdbstub: implement a softmmu based test

gdbstub: implement a softmmu based test

This adds a new tests that allows us to test softmmu only features
including watchpoints. To do achieve this we need to:

  - add _exit: labels to the boot codes
  - write a memory.py test case
  - plumb the test case into the build system
  - tweak the run_test script to:
    - re-direct output when asked
    - use socket based connection for all tests
    - add a small pause before connection

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210108224256.2321-6-alex.bennee@linaro.org>