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.
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