torture: Make torture.sh --allmodconfig testing fail on warnings

Currently, the torture.sh --allmodconfig testing looks solely at the
exit code from the kernel build, and thus fails to flag many compiler
warnings.  This commit therefore checks the kernel-build output for
compiler diagnostics.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>