Fix ThreadNameDoesNotAffectProcessName for Mono

[lambdageek]

See #34064

We need a check in mono_native_thread_set_name on Linux that the tid does not represent the main thread of the program (in the sense that on Linux gettid () == getpid () for the main thread).

The problem is that mono_native_thread_set_name gets a MonoNativeThreadId (which is pthread_t on Linux) and we need to compare that to the OS thread id of the main thread (which is a pid_t - gettid () of the main thread is the same as getpid()).

We have mono_native_thread_os_id_get () but it only returns the pid_t of the current thread - not of an arbitrary MonoNativeThreadId.

So we need some mechanism to go from a MonoNativeThreadId (or maybe a MonoThreadInfo) to a native OS thread id.

[lambdageek]

We have MonoThreadInfo *mono_thread_info_lookup (MonoNativeThreadId tid) so one way to do this is to add a guint64 os_id field to MonoThreadInfo.

The downside of mono_thread_info_lookup is it's a linear scan, which isn't great.

[lambdageek]

Hm. Actually a faster way might just be to add a global "main thread" MonoNativeThreadId and set it in register_thread when we see getpid () == mono_native_thread_os_id_get ()