twister: Implement GNU make jobserver support

When twister is started from make, use the information in MAKEFLAGS to
participate in the job server started by make.

When twister is running standalone, start a jobserver to share with
child processes. If using the kitware version of ninja, then this will
prevent ninja+twister from starting more processes than XX when twister
is run with -jXX.

There is no implementation for windows yet.

Signed-off-by: Jeremy Bettis <jbettis@google.com>