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/common/freertos/freertos_time.c

@@ -6,8 +6,15 @@
 #include "platform_api_vmcore.h"
 
 uint64
-os_time_get_boot_microsecond()
+os_time_get_boot_us()
 {
     TickType_t ticks = xTaskGetTickCount();
     return (uint64)1000 * 1000 / configTICK_RATE_HZ * ticks;
 }
+
+uint64
+os_time_thread_cputime_us(void)
+{
+    /* FIXME if u know the right api */
+    return os_time_get_boot_us();
+}

core/shared/platform/common/posix/posix_time.c

@@ -6,7 +6,7 @@
 #include "platform_api_vmcore.h"
 
 uint64
-os_time_get_boot_microsecond()
+os_time_get_boot_us()
 {
     struct timespec ts;
     if (clock_gettime(CLOCK_MONOTONIC, &ts) != 0) {
@@ -15,3 +15,14 @@ os_time_get_boot_microsecond()
 
     return ((uint64)ts.tv_sec) * 1000 * 1000 + ((uint64)ts.tv_nsec) / 1000;
 }
+
+uint64
+os_time_thread_cputime_us()
+{
+    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;
+}