incompatible pointee types in c-scape signatures

[nivkner]

as mentioned in #94, the following types and their libc counterparts do not match in layout in some architectures, and so should not used as is in pointers passed by the caller:

• rustix::fs::Stat: mismatch with libc::stat in i686
• SocketAddrStorage: mismatch with libc::sockaddr in i686
• PthreadAttrT: mismatch with libc::pthread_attr_t in aarch64
• PthreadMutexT: mismatch with libc::pthread_mutex_t in aarch64
• PthreadMutexattrT: mismatch with libc::pthread_mutexattr_t in aarch64

[Dan Gohman]

#120 fixes the pthread and sockaddr issues. That still leaves timespec/timeval and stat/stat64 on 32-bit platforms.

[Dan Gohman]

And the stat types were fixed in #104.