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/bh_log.c

@@ -31,7 +31,7 @@ bh_log(LogLevel log_level, const char *file, int line, const char *fmt, ...)
 
     self = os_self_thread();
 
-    usec = os_time_get_boot_microsecond();
+    usec = os_time_get_boot_us();
     t = (uint32)(usec / 1000000) % (24 * 60 * 60);
     h = t / (60 * 60);
     t = t % (60 * 60);

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