Fix building on older SBCL hosts

* Two parts here.  First, older SBCL hosts use SB-EXT:QUIT, not
SB-EXT:EXIT.  Second, there's some odd undefined-function warning
that crops up (probably from an inlined TYPEP test or even an
implicit type test) on the host from (SATISFIES FDEFN-P).

  * This doesn't get the parallel build working, but at least the
more usual single-threaded build works on 1.0.23 with these
changes.

loader.lisp

@@ -1,6 +1,9 @@
 (defun load-sbcl-file (file)
   (labels ((exit-sbcl (code)
-             #+sbcl (sb-ext:exit :code code)
+             #+sbcl #.(if (eq :external
+                              (nth-value 1 (find-symbol "EXIT" :sb-ext)))
+                          `(,(find-symbol "EXIT" :sb-ext) :code code)
+                          `(,(find-symbol "QUIT" :sb-ext) :unix-status code))
              #+ccl (ccl:quit code)
              #+abcl (ext:quit :status code)
              #+cmucl (unix:unix-exit code)

src/code/cross-misc.lisp

@@ -198,3 +198,8 @@
 
 ;;; Avoid an unknown type reference from globaldb.
 (deftype fdefn () '(satisfies fdefn-p))
+
+;;; Avoid an unknown function reference from globaldb on some build
+;;; hosts.  It doesn't really matter what this function does: we don't
+;;; have FDEFN objects on the host anyway.
+(defun fdefn-p (x) (declare (ignore x)) nil)