Generate binding files in parallel

Binding-generated files are standalone in that the process of
generating its source does not have any other side effects.

Improve the performance by generating these files in parallel
using the common ForkJoin pool -- thus taking advantage of all
available cores.

Change-Id: I4ee9efea0371fe9574721701ea9674793209cf24
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit a6fb0b3903e8630df18b7c63a116bfd06a716bdf)