Only initialize the part of the jobs array that will get used

The jobs array is getting initialized in O(compiled cpus^2) complexity.
Distros and people with bigger systems will use pretty high values
(128 or 256 or more) for this value, leading to interesting bubbles
in performance.

Baseline (single threaded performance) gets roughly 13 - 15 multiplications per cycle
in the interesting range (threading kicks in at 65x65 mult by 65x65).
The hardware is capable of 32 multiplications per cycle theoretically.

   Matrix          SGEMM cycles    MPC                                   DGEMM cycles      MPC
  48 x 48               10703.9   10.6       0.0%                             17990.6      6.3       0.0%
  64 x 64               20778.4   12.8       0.0%                             40629.2      6.5       0.0%
  65 x 65               26869.9   10.3       0.0%                             52545.7      5.3       0.0%
  80 x 80               38104.5   13.5       0.0%                             72492.7      7.1       0.0%
  96 x 96               61626.4   14.4       0.0%                            113983.8      7.8       0.0%
 112 x 112              91803.8   15.3       0.0%                            180987.3      7.8       0.0%
 128 x 128             133161.4   15.8       0.0%                            258374.3      8.1       0.0%

When threading is turned on
TARGET=SKYLAKEX F_COMPILER=GFORTRAN  SHARED=1 DYNAMIC_THREADS=1 USE_OPENMP=0  NUM_THREADS=128

  Matrix          SGEMM cycles    MPC                                   DGEMM cycles      MPC
  48 x 48               10725.9   10.5      -0.2%                             18134.9      6.2      -0.8%
  64 x 64               20500.6   12.9       1.3%                             40929.1      6.5      -0.7%
  65 x 65             2040832.1    0.1   -7495.2%                           2097633.6      0.1   -3892.0%
  80 x 80             2063129.1    0.2   -5314.4%                           2119925.2      0.2   -2824.3%
  96 x 96             2070374.5    0.4   -3259.6%                           2173604.4      0.4   -1806.9%
 112 x 112            2111721.5    0.7   -2169.6%                           2263330.8      0.6   -1170.0%
 128 x 128            2276181.5    0.9   -1609.3%                           2377228.9      0.9    -820.1%

There is a deep deep cliff once you hit 65x65

With this patch

   Matrix          SGEMM cycles    MPC                                   DGEMM cycles      MPC
  48 x 48               10630.0   10.6       0.7%                             18112.8      6.2      -0.7%
  64 x 64               20374.8   13.0       1.9%                             40487.0      6.5       0.4%
  65 x 65              141955.2    1.9    -428.3%                            146708.8      1.9    -179.2%
  80 x 80              178921.1    2.9    -369.6%                            186032.7      2.8    -156.6%
  96 x 96              205436.2    4.3    -233.4%                            224513.1      3.9     -97.0%
 112 x 112             244408.2    5.8    -162.7%                            262158.7      5.4     -47.1%
 128 x 128             321334.5    6.5    -141.3%                            333829.0      6.3     -29.2%

The cliff is very significantly reduced.
(more to follow)

driver/level3/level3_thread.c

@@ -658,8 +658,8 @@ static int gemm_driver(blas_arg_t *args, BLASLONG *range_m, BLASLONG
     }
 
     /* Clear synchronization flags */
-    for (i = 0; i < MAX_CPU_NUMBER; i++) {
+    for (i = 0; i < nthreads; i++) {
-      for (j = 0; j < MAX_CPU_NUMBER; j++) {
+      for (j = 0; j < nthreads; j++) {
 	for (k = 0; k < DIVIDE_RATE; k++) {
 	  job[i].working[j][CACHE_LINE_SIZE * k] = 0;
 	}