Makefile: Ensure we don't run Sphinx in parallel for manpages

Sphinx will corrupt its doctree cache if we run two copies
of it in parallel. In commit 6bda415c10d966c8d3 we worked
around this by having separate doctrees for 'html' vs 'manpage'
runs. However now that we have more than one manpage produced
from a single manual we can run into this again when trying
to produce the two manpages.

Use the trick described in 'Atomic Rules in GNU Make'
https://www.cmcrossroads.com/article/atomic-rules-gnu-make
to ensure that we only run the Sphinx manpage builder once
for each manual, even if we're producing several manpages.
This fixes doctree corruption in parallel builds and also
avoids pointlessly running Sphinx more often than we need to.

(In GNU Make 4.3 there is builtin support for this, via
the "&:" syntax, but we can't wait for that to be available
in all the distros we support...)

The generic "one invocation for multiple output files"
machinery is provided as a macro named 'atomic' in rules.mak;
we then wrap this in a more specific macro for defining
the rule and dependencies for the manpages in a Sphinx
manual, to avoid excessive repetition.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200124162606.8787-2-peter.maydell@linaro.org

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