Auto merge of #35378 - Amanieu:rwlock_eagain, r=alexcrichton
Auto merge of #35378 - Amanieu:rwlock_eagain, r=alexcrichton

Handle RwLock reader count overflow

`pthread_rwlock_rdlock` may return `EAGAIN` if the maximum reader count overflows. We shouldn't return a successful lock in that case.

graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust