rcutorture: Add ftrace-compatible timestamp to GP# failure/close-call output

This commit adds an ftrace-compatible microsecond-scale timestamp
to the failure/close-call output, but only in kernels built with
CONFIG_RCU_TORTURE_TEST_LOG_GP=y.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>