gh-120321: Make gi_frame_state transitions atomic in FT build

[colesbury]

This makes generator frame state transitions atomic in the free threading build, which avoids segfaults when trying to execute a generator from multiple threads concurrently.

There are still a few operations that aren't thread-safe and may crash if performed concurrently on the same generator/coroutine:

• Accessing gi_yieldfrom/cr_await/ag_await
• Accessing gi_frame/cr_frame/ag_frame
• Async generator operations

• Issue: SIGSEGV with generators in free-threaded build #120321