Add `OPENBLAS_DEFAULT_NUM_THREADS`

[staticfloat]

This allows Julia to set a default number of threads (usually 1) to be used when no other thread counts are specified [0], to short-circuit the default OpenBLAS thread initialization routine that spins up a different number of threads than Julia would otherwise choose.

The reason to add a new environment variable is that we want to be able to configure OpenBLAS to avoid performing its initial memory allocation/thread startup, as that can consume significant amounts of memory, but we still want to be sensitive to legacy codebases that set things like OMP_NUM_THREADS or GOTOBLAS_NUM_THREADS. Creating a new environment variable that is openblas-specific and is not already publicly used to control the overall number of threads of programs like Julia seems to be the best way forward.

[0] https://github.com/JuliaLang/julia/pull/46844