Auto merge of #633 - main--:rwlockattr, r=alexcrichton

Add pthread_rwlockattr APIs

I'm basically trying to mirror the existing code for pthread_mutexattr.

Hopefully the non-linux parts are correct - finding the right definitions is rather confusing and occasionally even contradicts existing definitions (e.g. [this](https://github.com/practicalswift/osx/blob/a26375e7b3e01c83e26a23977f909438ed9b31b4/src/libpthread/src/internal.h#L295) `pthread_condattr_t` doesn't match #335).