tools/memory-model: Document categories of ordering primitives

The Linux kernel has a number of categories of ordering primitives, which
are recorded in the LKMM implementation and hinted at by cheatsheet.txt.
But there is no overview of these categories, and such an overview
is needed in order to understand multithreaded LKMM litmus tests.
This commit therefore adds an ordering.txt as well as extracting a
control-dependencies.txt from memory-barriers.txt.  It also updates the
README file.

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

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