When determining the frame ID of an inline frame, GDB currently asserts
that a valid ID of the underlying real frame is found, and that it does
not match outer_frame_id. From inline_frame_this_id():
/* For now, require we don't match outer_frame_id either (see
comment above). */
gdb_assert (!frame_id_eq (*this_id, outer_frame_id));
However, this assertion may fail when the real frame's unwinder can not
determine the frame ID. This happened on an s390x target with a binary
that lacked call frame information and also confused the prologue
analyzer, because then s390_frame_this_id() left the frame ID at its
default.
To fix this, this change enhances s390_frame_this_id such that an
unavailable-stack frame ID is built if no frame base can be determined
but the function address is available.