The sizes of vmlinux files built with KASAN enabled can approach a full
gigabyte, which can result in disk overflow sooner rather than later.
Fortunately, the xz command compresses them by almost an order of
magnitude.  This commit therefore uses xz to compress vmlinux file built
by torture.sh with KASAN enabled.

However, xz is not the fastest thing in the world.  In fact, it is way
slower than rotating-rust mass storage.  This commit therefore also adds a
--compress-kasan-vmlinux argument to specify the degree of xz concurrency,
which defaults to using all available CPUs if there are that many files in
need of compression.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>