libstdc++: Write timestamp to libstdc++-performance.sum file

The results of 'make check-performance' are appended to the .sum file,
with no indication where one set of results ends and the next begins. We
could just remove the file when starting a new run, but appending makes
it a little easier to compare with previous runs, without having to copy
and store old files.

This adds a header containing a timestamp to the file when starting a
new run.

libstdc++-v3/ChangeLog:

	* scripts/check_performance: Add timestamp to output file at
	start of run.

libstdc++-v3/scripts/check_performance

@@ -38,6 +38,9 @@ TESTS_FILE="testsuite_files_performance"
 
 echo CXX is $CXX
 
+
+echo "=== $(date -u) ===" >> $BUILD_DIR/testsuite/libstdc++-performance.sum
+
 for NAME in `cat $TESTS_FILE`
 do
   RUN=true