Fix bit-position-overrun on 32-bit.

Apply mask-signed-field to the address or it won't be a fixnum for
higher addresses.

tests/bit-vector.impure-cload.lisp

@@ -133,7 +133,10 @@
  (setf (sb-sys:sap-ref-word addr sb-vm:n-word-bytes)
  (ash n-bits sb-vm:n-fixnum-tag-bits))
  (multiple-value-bind (object widetag size)
- (sb-vm::reconstitute-object (ash (sb-sys:sap-int addr) (- sb-vm:n-fixnum-tag-bits)))
+ (sb-vm::reconstitute-object (sb-c::mask-signed-field
+ sb-vm:n-fixnum-bits
+ (ash (sb-sys:sap-int addr)
+ (- sb-vm:n-fixnum-tag-bits))))
  (declare (ignore widetag))
  (assert (simple-bit-vector-p object))
  (assert (= size n-bytes))