xlat/gen.sh: speedup xlat generation

xlat/gen.sh: speedup xlat generation

This commit implements the following changes:
- increases the number of concurrent jobs to twice the CPU count;
- creates a circular buffer, so instead of running multiple jobs
  at once, the generator tries to keep about the same number of jobs
  being run concurrently;
- runs gen_git and gen_make concurrently in order to squeeze in
  one more bit of concurrency.

With my deeply scientific measurements, this approach achieves
up to 15% speedup with dash and about 30-40% with bash as /bin/sh
on a 4-core machine.

* xlat/gen.sh (main): Declare pids local variable, append pid of every
run job to it; increase the limit of jobs to ncpus * 2; when the limit
is reached, wait for the first pid in pids instead of resetting jobs
to zero and waiting for all the run jobs; put gen_git and gen_make
into background.