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.
before the change, only support wasm app exit like:
```c
void *thread_routine(void *arg)
{
printf("Enter thread\n");
return NULL;
}
```
if call pthread_exit, it will crash:
```c
void *thread_routine(void *arg)
{
printf("Enter thread\n");
pthread_exit(NULL);
return NULL;
}
```
This commit lets both upstairs work correctly, test pass on stm32f103 mcu.
