Store thread name

[jethrogb]

libstd/sys/sgx/thread.rs

    pub fn set_name(_name: &CStr) {
        // FIXME: could store this pointer in TLS somewhere
    }

[vn971]

PR: rust-lang/rust#112646

As a result of the investigation, I've concluded that the issue is a result of a wrong assumption:
It was thought that the thread name needs to be saved in the method Thread::set_name. However, it seems that this method has a very different purpose: it signals the platform-specific (target-specific) code to set the thread name in the operating system.
However, in SGX, there is no strict concept of an operating system thread name. And the name of the thread, as visible inside SGX, is already stored by the platform-agnostic Rust code.
Therefore, the only action to take here is to document the situation and remove the FIXME marker.

[jethrogb]

Apparently already implemented