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/iwasm/libraries/libc-builtin/libc_builtin_wrapper.c

@@ -927,7 +927,7 @@ clock_gettime_wrapper(wasm_exec_env_t exec_env, uint32 clk_id,
     if (!validate_native_addr(ts_app, sizeof(struct timespec_app)))
         return (uint32)-1;
 
-    time = os_time_get_boot_microsecond();
+    time = os_time_get_boot_us();
     ts_app->tv_sec = time / 1000000;
     ts_app->tv_nsec = (time % 1000000) * 1000;
 
@@ -939,7 +939,7 @@ clock_wrapper(wasm_exec_env_t exec_env)
 {
     /* Convert to nano seconds as CLOCKS_PER_SEC in wasi-sdk */
 
-    return os_time_get_boot_microsecond() * 1000;
+    return os_time_get_boot_us() * 1000;
 }
 
 #if WASM_ENABLE_SPEC_TEST != 0