cpu: convert queued work to a QSIMPLEQ

cpu: convert queued work to a QSIMPLEQ

We convert queued work to a QSIMPLEQ, instead of
open-coding it.

While at it, make sure that all accesses to the list are
performed while holding the list's lock.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Robert Foley <robert.foley@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200609200738.445-3-robert.foley@linaro.org>
Message-Id: <20200612190237.30436-6-alex.bennee@linaro.org>