torture: Make the build machine control N in "make -jN"

Given remote rcutorture runs, it is quite possible that the build system
will have fewer CPUs than the system(s) running the actual test scenarios.
In such cases, using the number of CPUs on the test systems can overload
the build system, slowing down the build or, worse, OOMing the build
system.  This commit therefore uses the build system's CPU count to set
N in "make -jN", and by tradition sets "N" to double the CPU count.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>