Auto merge of #100581 - joboet:sync_rwlock_everywhere, r=thomcc

std: use `sync::RwLock` for internal statics

Since `sync::RwLock` is now `const`-constructible, it can be used for internal statics, removing the need for `sys_common::StaticRwLock`. This adds some extra allocations on platforms which need to box their locks (currently SGX and some UNIX), but these will become unnecessary with the lock improvements tracked in #93740.

Author: bors