The pthread API is considered a standard feature of libc so better to
simply merge it with the libc. Pthreads are in fact already a part of
the libc in the form of weak symbols. This merger is also a prerequisite
for better integrating pthreads with the libc I/O task.
Fix#3054
By building the posix library as shared object with an ABI, we
effectively decouple posix-using programs from the library
implementation (which happens to depend on several os-level APIs such as
the VFS).