Add idle kernel

Only spawns 1 thread that does a busy wait up to a counter.  Other cores
do not issue any instructions after the scheduling prologue.

tests/regression/idle/kernel.cpp

@@ -0,0 +1,38 @@
+#include <stdint.h>
+#include <vx_intrinsics.h>
+#include <vx_print.h>
+#include <vx_spawn.h>
+#include "common.h"
+#include "include/gemmini.h"
+#include "gemmini_mmio.h"
+
+#define NUM_CLUSTERS 1
+#define NUM_THREADS_IN_CLUSTER 256
+
+#define HW_TID() ({uint32_t gtid; asm volatile ("csrr %0, mhartid" : "=r" (gtid)); gtid;})
+
+void kernel_body(int task_id, kernel_arg_t *__UNIFORM__ arg) {
+  constexpr uint32_t timer = 50000;
+  uint32_t counter = 0;
+  while ((counter++) < timer) {
+    asm("");
+  }
+  // to prevent optimize-out
+  // reinterpret_cast<uint32_t *>(arg->addr_c)[0] = counter;
+
+  vx_tmc(0);
+}
+
+int main() {
+  kernel_arg_t *arg = (kernel_arg_t *)KERNEL_ARG_DEV_MEM_ADDR;
+
+  // const uint32_t num_threads_in_cluster = NUM_THREADS_IN_CLUSTER;
+  // const uint32_t grid_size = num_threads_in_cluster * NUM_CLUSTERS;
+  const uint32_t grid_size = 1;
+#ifdef RADIANCE
+  vx_spawn_tasks_cluster(grid_size, (vx_spawn_tasks_cb)kernel_body, arg);
+#else
+  vx_spawn_tasks_contiguous(grid_size, (vx_spawn_tasks_cb)kernel_body, arg);
+#endif
+  return 0;
+}