[Mono] Don't set thread name of main thread on Linux (#36116)

[Mono] Don't set thread name of main thread on Linux (#36116)

* [mono] Record MonoNativeThreadId of the main thread

We would like to know the MonoNativeThreadId (pthread_t on Linux) of the main
thread of the application.  We can identify the main thread (on linux) because
it is the one for which `gettid () == getpid ()`.  (`gettid()` returns a
`pid_t` which is not the same thing as a `pthread_t`, hence this roundabout way
of detecting it.)

A complication arises in embedding scenarios: the main thread is not
necessarily the one that calls `mono_jit_init` or otherwise interacts with the
runtime.  Therefore we do the `gettid() == getpid ()` test at `MonoThreadInfo`
creation time when we call `register_thread`.

If the main thread never interacts with Mono, the main thread is not known to
us.

* [mono] Don't set name of main thread on Linux

Setting the name of the main thread also changes the name of the process.

Fixes https://github.com/dotnet/runtime/issues/35908

The corresponding fix for CoreCLR is https://github.com/dotnet/runtime/pull/34064

* Re-enable test from https://github.com/dotnet/runtime/pull/34064