error: cannot initialize return object of type 'uint64_t' (aka 'unsigned long') with an rvalue of type 'pthread_t'

**Describe the bug (描述bug)**
```
/usr/ports/devel/brpc/work/incubator-brpc-1.3.0/src/butil/compat.h:85:12: error: cannot initialize return object of type 'uint64_t' (aka 'unsigned long') with an rvalue of type 'pthread_t' (aka 'pthread *')
return pthread_self();
^~~~~~~~~~~~~~
2 errors generated.
``

**To Reproduce (复现方法)**
build on FreeBSD

According to the documentation pthread_self returns pthread_t:
```
NAME
pthread_self – get the calling thread's ID

LIBRARY
POSIX Threads Library (libpthread, -lpthread)

SYNOPSIS
#include

pthread_t
pthread_self(void);
```
pthread_t generally can not and should not be typecast to any other type.

**Versions (各种版本)**
OS: FreeBSD 13.1
Compiler: clang-14
brpc: 1.3.0

Start with src/butil/compat.h at line 85 and reproduce the build on FreeBSD 13.1 with clang-14, as described in the issue. Check the pthread_self return type and the surrounding compatibility code; done means brpc 1.3.0 builds without this error on FreeBSD.

Written by the indexing model from the issue text.