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/platform/rt-thread/rtt_platform.c

@@ -120,13 +120,20 @@ os_vprintf(const char *format, va_list ap)
 }
 
 uint64
-os_time_get_boot_microsecond(void)
+os_time_get_boot_us(void)
 {
     uint64 ret = rt_tick_get() * 1000;
     ret /= RT_TICK_PER_SECOND;
     return ret;
 }
 
+uint64
+os_time_thread_cputime_us(void)
+{
+    /* FIXME if u know the right api */
+    return os_time_get_boot_us();
+}
+
 korp_tid
 os_self_thread(void)
 {