[gdb/testsuite] Add make-check-all.sh

[gdb/testsuite] Add make-check-all.sh

Directory gdb/testsuite/boards contains a number of host/target boards, which
run a test-case (or test-cases) in a different way.

The benefits of using these boards are:
- improving test coverage of gdb,
- making the testsuite more robust, and
- making sure the test-cases work for non-native and remote setups, if
  possible.

Each board is slightly different, and developers need to learn how to use each
one, what parameters to pass and how, and which ones can be used in
combination with each other.  This is a threshold to start using them.

And then there quite a few, so I suppose typically only a few will be used by
each developer.

Add script gdb/testsuite/make-check-all.sh, that's intended to function as a
drop-in replacement of make check, while excercising all host/target boards in
gdb/testsuite/boards.

An example of make-check-all.sh for one test-case is:
...
 $  ~/gdb/src/gdb/testsuite/make-check-all.sh gdb.base/advance.exp
 LOCAL:
 # of expected passes            8
 TARGET BOARD: cc-with-gdb-index
 # of expected passes            8
   ...
 HOST BOARD: local-remote-host-notty, TARGET BOARD: remote-stdio-gdbserver
 # of expected passes            8
 HOST/TARGET BOARD: local-remote-host-native
 # of expected passes            8
...

Shell-checked and tested on x86_64-linux.

Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
Reviewed-By: Andrew Burgess <aburgess@redhat.com>