Port Qnil==0 changes to 32-bit --with-wide-int

Port Qnil==0 changes to 32-bit --with-wide-int

* lisp.h (lisp_h_XSYMBOL, XSYMBOL): Assume USE_LSB_TAG in the
macro-implemented version.  For the non-USE_LSB_TAG case, supply
a new inline function that is the inverse of the new TAG_SYMPTR.
(lisp_h_XUNTAGBASE, XUNTAGBASE): Remove.  All uses removed.
(TAG_SYMPTR) [!USE_LSB_TAG]: If the pointer subtraction yields a
negative number, don't allow sign bits to bleed into the encoded
value.  Shift in zero bits instead.