uek-rpm: build: sign modules in parallel

uek-rpm: build: sign modules in parallel

Assuming you have enough entropy, this gives a speedup in the
signing phase almost precisely proportional to the core count.

e.g. on a 20-core box (with %_smp_ncpus_max forced to 0 in
/etc/rpm/macros, but this will be irrelevant shortly, and even with the
present RPM configuration a 16-fold speedup can be seen):

old:
168.27user 19.99system 3:06.25elapsed 101%CPU (0avgtext+0avgdata 11112maxresident)k

new:
138.70user 11.32system 0:10.14elapsed 1479%CPU (0avgtext+0avgdata 11208maxresident)k

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: John Sobecki <john.sobecki@oracle.com>
Orabug: 26316447