Enhancements on wasm function execution time statistic (#2985)

Enhance the statistic of wasm function execution time, or the performance
profiling feature:
- Add os_time_thread_cputime_us() to get the cputime of a thread,
  and use it to calculate the execution time of a wasm function
- Support the statistic of the children execution time of a function,
  and dump it in wasm_runtime_dump_perf_profiling
- Expose two APIs:
  wasm_runtime_sum_wasm_exec_time
  wasm_runtime_get_wasm_func_exec_time

And rename os_time_get_boot_microsecond to os_time_get_boot_us.

core/iwasm/libraries/thread-mgr/thread_manager.c

@@ -667,6 +667,12 @@ thread_manager_start_routine(void *arg)
            since we will exit soon */
     }
 
+#if WASM_ENABLE_PERF_PROFILING != 0
+    os_printf("============= Spawned thread ===========\n");
+    wasm_runtime_dump_perf_profiling(module_inst);
+    os_printf("========================================\n");
+#endif
+
     /* Free aux stack space */
     free_aux_stack(exec_env, exec_env->aux_stack_bottom.bottom);
     /* Remove exec_env */