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/include/platform_api_vmcore.h

@@ -64,7 +64,13 @@ os_vprintf(const char *format, va_list ap);
  * Get microseconds after boot.
  */
 uint64
-os_time_get_boot_microsecond(void);
+os_time_get_boot_us(void);
+
+/**
+ * Get thread-specific CPU-time clock in microseconds
+ */
+uint64
+os_time_thread_cputime_us(void);
 
 /**
  * Get current thread id.