Fix build with SBCL 2.4.7 on x86-32

It looks like the %add-with-carry and %subtract-with-borrow symbols are not
available anymore in the sb-bignum package.

src/common.lisp

@@ -766,12 +766,6 @@ uint64_t r = (n << 56)
 (defun %add-with-carry (x y carry)
   (declare (type (unsigned-byte 32) x y)
            (type (mod 2) carry))
-  #+(and sbcl 32-bit)
-  (sb-bignum:%add-with-carry x y carry)
-  #+(and cmucl 32-bit)
-  (bignum:%add-with-carry x y carry)
-  #-(or (and sbcl 32-bit)
-        (and cmucl 32-bit))
   (let* ((temp (mod32+ x y))
          (temp-carry (if (< temp x) 1 0))
          (result (mod32+ temp carry)))
@@ -780,12 +774,6 @@ uint64_t r = (n << 56)
 (defun %subtract-with-borrow (x y borrow)
   (declare (type (unsigned-byte 32) x y)
            (type (mod 2) borrow))
-  #+(and sbcl 32-bit)
-  (sb-bignum:%subtract-with-borrow x y borrow)
-  #+(and cmucl 32-bit)
-  (bignum:%subtract-with-borrow x y borrow)
-  #-(or (and sbcl 32-bit)
-        (and cmucl 32-bit))
   (let ((temp (mod32- x y)))
     (cond
       ((zerop borrow)