Avoid exceeding the configured thread count in x86_64 TOBF16 (#4748)

* avoid setting nthreads higher than available

kernel/x86_64/tobf16.c

@@ -144,10 +144,11 @@ void CNAME(BLASLONG n, FLOAT_TYPE * in, BLASLONG inc_in, bfloat16 * out, BLASLON
     if (inc_in == 0 || inc_out == 0 || n <= 100000) {
         nthreads = 1;
     } else {
+        nthreads = num_cpu_avail(1);
         if (n/100000 < 100) {
-            nthreads = 4;
-        } else {
-            nthreads = 16;
+            nthreads = MAX(nthreads,4);
+//        } else {
+//            nthreads = MAX(nthreads,16);
         }
     }