Recreate the pathname intern table on save

Don't save a hash-table kv vector full of a zillion unbound markers.
Some other tables might want the same treatment but I'm not sure which.

src/code/save.lisp

@@ -339,9 +339,13 @@ sufficiently motivated to do lengthy fixes."
   (profile-deinit)
   (foreign-deinit)
   ;; To have any hope of making pathname interning actually work,
-  ;; this CLRHASH would need to be removed. But removing it causes excess
-  ;; garbage retention because weakness doesn't work. It's a catch-22.
-  (clrhash *pathnames*)
+  ;; this MAKE-PATHNAME-TABLE (formerly CLRHASH) would need to be removed.
+  ;; But removing this step causes excess garbage retention because the
+  ;; table's weakness isn't actually working (not sure why).
+  ;; And CLRHASH doesn't do as much as it should, because if the table
+  ;; is actually empty, but at some point held 20,000 pathnames,
+  ;; then we'd write out an enormous vector full of nothing.
+  (make-pathname-intern-table)
   ;; Clean up the simulated weak list of covered code components.
   (rplacd sb-c:*code-coverage-info*
           (delete-if-not #'weak-pointer-value (cdr sb-c:*code-coverage-info*)))

src/code/target-pathname.lisp

@@ -298,7 +298,7 @@
 ;;; *around* the get and put operations. It makes no sense to lock the table around
 ;;; individual operations, hence the unsynchronized table.
 
-(define-load-time-global *pathnames*
+(defun make-pathname-intern-table ()
     (let ((h (%make-hash-table (logior (pack-ht-flags-weakness +ht-weak-value+)
                                        (pack-ht-flags-kind 3)
                                        hash-table-userfun-flag)
@@ -310,6 +310,8 @@
                                $1.0)))
       (install-hash-table-lock h)
       h))
+(define-load-time-global *pathnames* (make-pathname-intern-table))
+
 ;;; A pathname is logical if the host component is a logical host.
 ;;; This constructor is used to make an instance of the correct type
 ;;; from parsed arguments.