[HUST CSE] Add a useless return value for _thread_entry to avoid compilation error.

[Hornos3]

The file /arch/linux/posix/gcc/port.c has a function _thread_entry, which has no return value but defined as void*(void), to avoid compilation error, a 'return NULL' was added into the end of this function.

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}