Prevent OpenBLAS from modifying the scheduler affinity.

Julia already manages the scheduler affinity by itself, which is subsequently
undone by OpenBLAS pinning to the first core available. Because of this, external
commands spawned from Julia would be restricted to a single core, which in case of
multithreaded applications is very suboptimal.
Merge pull request #3097 from maleadt/master

Prevent OpenBLAS from modifying the scheduler affinity