math/openblas: Increase the limit for the number of threads to unbreak other ports

NUM_THREADS= sets the build-time limit on the number of threads that apps can use with OpenBlas.
This unbreaks at least science/erkale's tests, and possibly some other software.
The upstream acknowledges the problem, recommended this solution for the port,
and are working on the permanent solution: https://github.com/xianyi/OpenBLAS/issues/1882

Approved by: portmgr blanket (unbreak)