Fix SBCL on big-endian backported from the unreleased sbcl-2.6.8.

From <https://github.com/sbcl/sbcl/commit/d28fd45be76a37fdb0106527aae61b5592f271b5>.

Suggested by <https://github.com/barracuda156> in <https://github.com/macports/macports-ports/pull/33998#issuecomment-5268699254>.


diff --git src/compiler/constraint.lisp src/compiler/constraint.lisp
index 492ea442b8..fde14b0e61 100644
--- src/compiler/constraint.lisp
+++ src/compiler/constraint.lisp
@@ -530,14 +530,17 @@
                              (aref ,universe
                                    ;; COUNT-TRAILING-ZEROS produces slightly better code than
                                    ;; INTEGER-LENGTH.  Either iteration direction is ok.
-                                   (logior (sb-c::if-vop-existsp (:translate count-trailing-zeros)
-                                             (prog1 (count-trailing-zeros ,word)
-                                               ;; Clear the lowest 1 bit via the Brian Kernighan
-                                               ;; technique (allegedly)
-                                               (setq ,word (logand ,word (sb-vm::+-mod64 ,word -1))))
-                                             (let ((bit (1- (integer-length ,word))))
-                                               (setq ,word (logxor ,word (ash 1 bit)))
-                                               bit))
+                                   (logior (let ((bit
+                                                   (sb-c::if-vop-existsp (:translate count-trailing-zeros)
+                                                     (prog1 (count-trailing-zeros ,word)
+                                                       ;; Clear the lowest 1 bit via the Brian Kernighan
+                                                       ;; technique (allegedly)
+                                                       (setq ,word (logand ,word (sb-vm::+-mod64 ,word -1))))
+                                                     (let ((bit (1- (integer-length ,word))))
+                                                       (setq ,word (logxor ,word (ash 1 bit)))
+                                                       bit))))
+                                             #+little-endian bit
+                                             #+big-endian (- sb-vm:n-word-bits 1 bit))
                                            (* ,index sb-vm:n-word-bits))))))
                       ,@body)))
              finally (return ,result)))