macOS OpenMP critically slow in specific circumstances
[Benchmark file.](https://github.com/mratsim/Arraymancer/blob/853e4f01b14f8c9a3a3851c8c998e098506fd6d5/benchmarks/implementation/logsumexp.nim)

Results on i5-5257U (dual-core mobile 2.7GHz, turbo 3.1)
Batch size 256

Compilation flag no OpenMP
`nim c -d:native -d:release --out:bin/logsumexp --nimcache:./nimcache benchmarks/implementation/logsumexp.nim`

Compilation flag OpenMP
`nim c d:openmp -d:native -d:release --out:bin/logsumexp --nimcache:./nimcache benchmarks/implementation/logsumexp.nim`

| Implementation | Classes | OpenMP | epochTime (seconds)      |
| -------------- | ------- | ------ | ------------------------ |
| Classic        | 1000    | No     | 1.869992971420288        |
| Streaming      | 1000    | No     | 1.463253974914551        |
| Classic        | 3       | No     | 0.005753040313720703     |
| Streaming      | 3       | No     | 0.004420995712280273     |
| Classic        | 1000    | Yes    | **2.416302919387817**    |
| Streaming      | 1000    | No*    | 1.478353023529053        |
| Classic        | 3       | Yes    | **0.005849123001098633** |
| Streaming      | 3       | No*    | 0.004467964172363281     |


No*: the streaming logsumexp is compiled with but does not use OpenMP