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/shared/utils/runtime_timer.c

@@ -38,7 +38,7 @@ struct _timer_ctx {
 uint64
 bh_get_tick_ms()
 {
-    return os_time_get_boot_microsecond() / 1000;
+    return os_time_get_boot_us() / 1000;
 }
 
 uint32