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.
diff --git a/gdb/testsuite/make-check-all.sh b/gdb/testsuite/make-check-all.sh
index 947e6e3..e0c1fdb 100755
--- a/gdb/testsuite/make-check-all.sh
+++ b/gdb/testsuite/make-check-all.sh
@@ -187,6 +187,10 @@
 
 	mkdir -p "$dir"
 	cp gdb.sum gdb.log "$dir"
+
+	# Record the 'make check' command to enable easy re-running.
+	echo "make check RUNTESTFLAGS=\"${rtf[*]} ${tests[*]}\"" \
+	     > "$dir/make-check.sh"
     fi
 }