[gdb] Rename variable main_thread to main_thread_id

I noticed that the variable main_thread:
...
/* The main thread.  */

static std::thread::id main_thread;
...
has a confusing name and corresponding comment, because it doesn't contain the
main thread, but rather the main thread's std::thread::id.

Fix this by renaming to main_thread_id.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>