Explode atomic box ops to new atomic instructions

* libguile/instructions.c (FOR_EACH_INSTRUCTION_WORD_TYPE): Add C8_S24
  word type.
* libguile/vm-engine.c (UNPACK_8_24): New helper.
  (atomic-scm-ref/immediate, atomic-scm-set!/immediate)
  (atomic-swap-scm!/immediate, atomic-scm-compare-and-swap!/immediate):
  New instructions.
  (make-atomic-box, atomic-box-ref, atomic-box-set!, atomic-box-swap!)
  (atomic-box-compare-and-swap!): Disable these ops.
* module/language/bytecode.scm (compute-instruction-arity): Add C8_S24
  support.
* module/system/vm/assembler.scm: Add C8_S24 support.  Export assemblers
  for new opcodes.
* module/system/vm/disassembler.scm (disassembler): Support C8_S24.
* module/language/cps/compile-bytecode.scm (compile-function): Replace
  old atomic-box assemblers with the new instructions.
* module/language/cps/effects-analysis.scm (annotation->memory-kind):
* module/language/cps/types.scm (annotation->type): Add cases for atomic
  boxes.  Mark as all memory kinds because atomic ops serialize memory
  accesses.
* module/language/tree-il/compile-cps.scm (make-atomic-box):
  (ensure-atomic-box, atomic-box-ref, atomic-box-set!):
  (atomic-box-swap!, atomic-box-compare-and-swap!): Explode these ops to
  more basic instructions.
* module/system/base/types/internal.scm (%tc7-atomic-box): Add forgotten
  export.

module/language/bytecode.scm

@@ -63,6 +63,7 @@
       ((L32) 1)
       ((LO32) 1)
       ((C8_C24) 2)
+      ((C8_S24) 2)
       ((C16_C16) 2)
       ((B1_C7_L24) 3)
       ((B1_X7_S24) 2)