The arm64 hardware was especially hit by only building on 3 of the 32
available cores. Introduce a JOBS environment variable which we use
for all parallel builds. We still run the main checks single threaded
though so to make it easier to spot hangs.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200203090932.19147-12-alex.bennee@linaro.org>