gdb/testsuite: log 'make check' command in make-check-all.sh

gdb/testsuite: log 'make check' command in make-check-all.sh

I have been making more use of the make-check-all.sh script to run
tests against all boards.

But one thing is pretty annoying.  When a test fails on some random
board, I have to run make-check-all.sh with --verbose and --dry-run in
order to see what RUNTESTFLAGS I should be using.

I always run with --keep-results on, so, in this commit, I propose
that, when --keep-results is on the 'make check' command will be
written out to a file within the stored results directory, like:

  check-all/BOARD_NAME/make-check.sh

then, if I want to rerun a test, I can just:

  sh check-all/BOARD_NAME/make-check.sh

and the test will be re-run for me.