Rework backend-specific CPS lowering

* module/system/base/target.scm (target-runtime): New export.
* module/language/cps/optimize.scm (make-cps-lowerer): Load a
backend-specific lowering module dynamically.

* module/language/cps/guile-vm.scm: New module for lowering to Guile's
VM.
* module/language/cps/guile-vm/loop-instrumentation.scm:
* module/language/cps/guile-vm/lower-primcalls.scm:
* module/language/cps/guile-vm/reify-primitives.scm: Move here, from
parent dir.

* module/language/cps/hoot.scm: New module for lowering to Wasm/GC via
Hoot.
* module/language/cps/guile-vm/tailify.scm:
* module/language/cps/guile-vm/unify-returns.scm: Move here, from parent
dir.

* am/bootstrap.am: Update for new file list.

module/language/cps/guile-vm/loop-instrumentation.scm

@@ -0,0 +1,67 @@
+;;; Continuation-passing style (CPS) intermediate language (IL)
+
+;; Copyright (C) 2016, 2017, 2018, 2020, 2023 Free Software Foundation, Inc.
+
+;;;; This library is free software; you can redistribute it and/or
+;;;; modify it under the terms of the GNU Lesser General Public
+;;;; License as published by the Free Software Foundation; either
+;;;; version 3 of the License, or (at your option) any later version.
+;;;;
+;;;; This library is distributed in the hope that it will be useful,
+;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;;; Lesser General Public License for more details.
+;;;;
+;;;; You should have received a copy of the GNU Lesser General Public
+;;;; License along with this library; if not, write to the Free Software
+;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+;;; Commentary:
+;;;
+;;; A pass to add "instrument-loop" primcalls at loop headers.
+;;;
+;;; Code:
+
+(define-module (language cps guile-vm loop-instrumentation)
+  #:use-module (ice-9 match)
+  #:use-module (language cps)
+  #:use-module (language cps utils)
+  #:use-module (language cps with-cps)
+  #:use-module (language cps intmap)
+  #:use-module (language cps intset)
+  #:use-module (language cps renumber)
+  #:export (add-loop-instrumentation))
+
+(define (compute-loop-headers cps)
+  (define (maybe-add-header label k headers)
+    "Add K to headers if it is a target of a backward branch."
+    (if (<= k label)
+        (intset-add! headers k)
+        headers))
+  (define (visit-cont label cont headers)
+    (match cont
+      (($ $kargs names vars ($ $continue k))
+       (maybe-add-header label k headers))
+      (($ $kargs names vars ($ $branch kf kt))
+       (maybe-add-header label kf (maybe-add-header label kt headers)))
+      (($ $kargs names vars ($ $switch kf kt*))
+       (fold1 (lambda (k headers) (maybe-add-header label k headers))
+              (cons kf kt*)
+              headers))
+      (_ headers)))
+  (persistent-intset (intmap-fold visit-cont cps empty-intset)))
+
+(define (add-loop-instrumentation cps)
+  (define (add-instrumentation label cps)
+    (match (intmap-ref cps label)
+      (($ $kargs names vars term)
+       (with-cps cps
+         (letk k ($kargs () () ,term))
+         (setk label
+               ($kargs names vars
+                 ($continue k #f
+                   ($primcall 'instrument-loop #f ()))))))))
+  (let* ((cps (renumber cps))
+         (headers (compute-loop-headers cps)))
+    (with-fresh-name-state cps
+      (persistent-intmap (intset-fold add-instrumentation headers cps)))))