sphinx: Use separate doctree directories for different builders

sphinx: Use separate doctree directories for different builders

sphinx-build is buggy when multiple processes are using the same
doctree directory in parallel.  See the 3-year-old Sphinx bug
report at: https://github.com/sphinx-doc/sphinx/issues/2946

Instead of avoiding parallel builds or adding some kind of
locking, I'm using the simplest solution: just using a different
doctree cache for each builder.

Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>
Reviewed-by: John Snow <[email protected]>
Message-id: 20191014150133[email protected]
Signed-off-by: Peter Maydell <[email protected]>