Add OPENBLAS_DEFAULT_NUM_THREADS

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] JuliaLang/julia#46844

driver/others/init.c

@@ -823,6 +823,8 @@ void gotoblas_affinity_init(void) {
 
   if (numprocs == 0) numprocs = readenv_atoi("OMP_NUM_THREADS");
 
+  if (numprocs == 0) numprocs = readenv_atoi("OPENBLAS_DEFAULT_NUM_THREADS");
+
   numnodes = 1;
 
   if (numprocs == 1) {