run: Limit OMP to the -j threads limit.

Merged Emma Anholt requested to merge anholt/shader-db:run-j into master

The -j argument only worked for the "1" case where OMP is skipped. Otherwise, we'd spawn the max amount of threads (except for -j 1 where OMP is skipped) and overflow the current_shader_names array.

Noticed when trying to work around an apparent freedreno bug with too many contexts on the same screen.

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