Support rwlock's in pthread (IDFGH-5690)

[MabezDev]

As part of an ongoing effort to bring Rust support to Espressif chips, one requirement we found is the need for rwlock to fully support the Rust standard library. For our initial implementation was accepted on the basis that rwlock support would come to esp-idf soon ™️ (rust-lang/libc#2310 (comment)).

We are currently getting away with the lack of support with a nasty hack using a mutex. Obviously we want to get rid of this asap.

[0xjakob]

@MabezDev Any requirements regarding the implementation? In particular, is writer starvation prevention necessary?

[MabezDev]

From the Rust standard library on the RWLock's:

The priority policy of the lock is dependent on the underlying operating system’s implementation, and this type does not guarantee that any particular policy will be used. In particular, a writer which is waiting to acquire the lock in write might or might not block concurrent calls to read.

Therefore I don't think we need writer starvation prevention initially, and we are open to change it later if we wish too. I think for now the simplest implementation will do.