gh-158539: Fix exception mode missing handlers in generators/coroutines

The sampling profiler's exception mode decided whether a thread was
handling an exception by reading the embedded PyThreadState.exc_state.
Generators, coroutines and async generators repoint tstate->exc_info
at their own _PyErr_StackItem while they run, so an except block
running in one of them (or in a function they call) stored the
exception in that item instead, and was never sampled.

Follow tstate->exc_info and its previous_item chain, mirroring
_PyErr_GetTopmostException(), and export the two debug offsets needed
to walk the chain from remote memory.  The common case where exc_info
points at the embedded exc_state keeps the existing zero-extra-read
fast path.

Author: hetaozdh

Include/internal/pycore_debug_offsets.h

@@ -113,11 +113,13 @@ typedef struct _Py_DebugOffsets {
         uint64_t gil_requested;
         uint64_t current_exception;
         uint64_t exc_state;
+        uint64_t exc_info;
     } thread_state;
 
     // Exception stack item offset
     struct {
         uint64_t exc_value;
+        uint64_t previous_item;
     } err_stackitem;
 
     // InterpreterFrame offset;
@@ -304,9 +306,11 @@ typedef struct _Py_DebugOffsets {
         .gil_requested = offsetof(PyThreadState, gil_requested), \
         .current_exception = offsetof(PyThreadState, current_exception), \
         .exc_state = offsetof(PyThreadState, exc_state), \
+        .exc_info = offsetof(PyThreadState, exc_info), \
     }, \
     .err_stackitem = { \
         .exc_value = offsetof(_PyErr_StackItem, exc_value), \
+        .previous_item = offsetof(_PyErr_StackItem, previous_item), \
     }, \
     .interpreter_frame = { \
         .size = sizeof(_PyInterpreterFrame), \