S390: Add use of unavailable-stack frame ID

S390: Add use of unavailable-stack frame ID

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.

gdb/ChangeLog:

	* s390-linux-tdep.c (s390_prologue_frame_unwind_cache): Store
	frame func's PC in info->func before any other failure can occur.
	(s390_frame_this_id): Use frame_id_build_unavailable_stack if
	info->func has been filled out.