Refactor handling of auxiliary stacks and setjmp/longjmp

* libguile/__scm.h (scm_i_jmp_buf): Remove definition, which was a shim
  for ia64.  Instead, always use setjmp/longjmp and jmp_buf.
* libguile/_scm.h (SCM_I_SETJMP, SCM_I_LONGJMP): Remove; instead use
  setjmp and longjmp.
* libguile/continuations.c (capture_auxiliary_stack):
  (restore_auxiliary_stack): New helpers.
  (scm_i_make_continuation): Use capture_auxiliary_stack.
  (copy_stack_and_call): Use restore_auxiliary_stack.  No need to stash
  the aux stack on the thread, either.
* libguile/continuations.h (scm_t_contregs): Use
  SCM_HAVE_AUXILIARY_STACK to flag when to have an auxiliary_stack
  member.
* libguile/control.h:
* libguile/control.c (reify_partial_continuation, scm_c_abort):
  (scm_suspendable_continuation_p): Adapt to use setjmp/longjmp
  directly.
* libguile/deprecated.h: Add deprecated scm_i_jmp_buf define.
* libguile/dynstack.h:
* libguile/dynstack.c (PROMPT_JMPBUF):
  (scm_dynstack_push_prompt, scm_dynstack_find_prompt):
  (scm_dynstack_wind_prompt): Adapt to jmp_buf type.
* libguile/eval.c (eval): Use jmp_buf and setjmp directly.
* libguile/gc-malloc.c: No need for ia64-specific things.
* libguile/gc.c: No need for ia64-specific things.
* libguile/gc.h: No need to declare scm_ia64_ar_bsp.
* libguile/init.c: Remove typedef of setjmp_type for Cray, unused.
* libguile/threads.c (guilify_self_1): No more pending_rbs_continuation
  in scm_i_thread, and register_backing_store_base is handled by libgc.
  (scm_ia64_ar_bsp): Remove definitions; inlined into continuations.c's
  capture_auxiliary_stack.
* libguile/threads.h (scm_i_thread): jmpbuf member is plain jmp_buf.
* libguile/throw.c (catch): Just use jmp_buf and setjmp.
* libguile/vm-engine.c (VM_NAME): Adapt prototype to take jmp_buf
  pointer.
* libguile/vm.c (vm_abort): Adapt jmp_buf types.
  (scm_call_n): Use setjmp.

libguile/vm-engine.c

@@ -269,7 +269,7 @@
 
 static SCM
 VM_NAME (scm_i_thread *thread, struct scm_vm *vp,
-         scm_i_jmp_buf *registers, int resume)
+         jmp_buf *registers, int resume)
 {
   /* Instruction pointer: A pointer to the opcode that is currently
      running.  */