fix up log-file toggling

fix up log-file toggling

Currently a proc in gdb.exp toggles the expect (and thus dejagnu)
logging.  This is not a super idea, but it is there to avoid putting
some preprocessor output into the log.

In the right circumstances, this can result in the log file being
mysteriously truncated.  I think this happens because it doesn't
necessarily write to the correct log file again.

The fix is to use "log_file -info" to save the previous log file.

2013-11-04  Tom Tromey  <tromey@redhat.com>

	* lib/gdb.exp (get_compiler_info): Use log_file -info and
	restore from that.