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/alios/alios_time.c

@@ -6,7 +6,14 @@
 #include "platform_api_vmcore.h"
 
 uint64
-os_time_get_boot_microsecond()
+os_time_get_boot_us()
 {
     return (uint64)aos_now_ms() * 1000;
 }
+
+uint64
+os_time_thread_cputime_us(void)
+{
+    /* FIXME if u know the right api */
+    return os_time_get_boot_us();
+}