Naming of Mutex

Is there a particular reason the mutex is named `SgxMutex` rather than the standard name `Mutex` from the std? The name difference is particularly annoying when we want to use the same code compile to a standard Linux process and sgx application.

Start by locating the SDK's SgxMutex definition and its uses, then compare them with Rust std's Mutex naming and the Linux-versus-SGX compilation paths. Determine whether the names can be aligned without breaking existing callers, and verify that the same code can compile for both targets when done.

Written by the indexing model from the issue text.