gdb/testsuite: add simavr.exp board

gdb/testsuite: add simavr.exp board

This patch adds a board file for against a simavr target (so, for the
AVR architecture).

simavr, when started with option -g, runs a GDB stub on port 1234.  In
the current latest release (1.6), the port is hardcoded to 1234.  But in
master, there is the option to choose another port.  So while the board
file hardcodes the port today, in the future it should be possible to
let the user choose a port, or automatically select a free port.

It is easy enough to run, make sure you have avr-gcc/avr-g++ and simavr
installed, and as usual just run:

    make check RUNTESTFLAGS="--target_board=simavr"

The following environment variables influence the behavior of the board
file:

  - SIMAVR_MCU: type of chip to simulate
  - SIMAVR_PATH: path to simavr binary (useful if you build your own
    simavr or for some reason it is not simply called `simavr`.

As expected, there are a lot of failures.  Many tests use some features
not supported by such a target, and I suppose there are real GDB bugs
too.  But a lot also passes (including tests that actually run stuff),
so this board file should still help to validate changes to the AVR
architecture support.

These are the results I got of running tests gdb.base/*.exp:

    # of expected passes            20926
    # of unexpected failures        2257
    # of expected failures          14
    # of unknown successes          1
    # of known failures             13
    # of unresolved testcases       592
    # of untested testcases         156
    # of unsupported tests          30
    # of paths in test names        3
    # of duplicate test names       56

gdb/testsuite/ChangeLog:

	* boards/simavr.exp: New file.

Change-Id: Ib7fa8c4e2e90b08b104bb9b552df37779de3bc21