[OpenBLAS] Set GEMM_MULTITHREAD_THRESHOLD to 0

[jayhawk-commits]

Trying out multi-threaded for all GEMM sizes.

From https://github.com/OpenMathLib/OpenBLAS/blob/develop/Makefile.rule#L241-L246

# If any gemm argument m, n or k is less or equal this threshold, gemm will be execute
# with single thread. (Actually in recent versions this is a factor proportional to the
# number of floating point operations necessary for the given problem size, no longer
# an individual dimension). You can use this setting to avoid the overhead of multi-
# threading in small matrix sizes. The default value is 4, but values as high as 50 have 
# been reported to be optimal for certain workloads (50 is the recommended value for Julia).