.travis.yml: probe for number of available processors

.travis.yml: probe for number of available processors

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 <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20200203090932[email protected]>