torture: Better summary diagnostics for build failures

The reaction of kvm-recheck.sh is obscure at best, and easy to miss
completely.  This commit therefore prints "BUG: Build failed" in the
summary at the end of a run.  This commit also adds the line of dashes
in cases where performance info is not available, and also avoids
printing nonsense diagnostics in cases where some of the normal test
output is not available.  In addition, this commit saves off the .config
file even when the build fails.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>