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/linux-sgx/sgx_time.c

@@ -26,7 +26,7 @@ ocall_clock_nanosleep(int *p_ret, unsigned clock_id, int flags,
                       const void *rem_buf, unsigned int rem_buf_size);
 
 uint64
-os_time_get_boot_microsecond()
+os_time_get_boot_us()
 {
 #ifndef SGX_DISABLE_WASI
     struct timespec ts;
@@ -40,6 +40,21 @@ os_time_get_boot_microsecond()
 #endif
 }
 
+uint64
+os_time_thread_cputime_us(void)
+{
+#ifndef SGX_DISABLE_WASI
+    struct timespec ts;
+    if (clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts) != 0) {
+        return 0;
+    }
+
+    return ((uint64)ts.tv_sec) * 1000 * 1000 + ((uint64)ts.tv_nsec) / 1000;
+#else
+    return 0;
+#endif
+}
+
 #ifndef SGX_DISABLE_WASI
 
 int