Improve GDB symbol export.

Improve GDB symbol export.

* .gdbinit (xgetptr, xgetint, xgettype): Set $bugfix in different
arms of an 'if', not using conditional expressions; otherwise GDB
complains about the types in the unevaluated arm when the argument
is an integer literal.
(xgetint): Simplify expression.
* alloc.c (gdb_make_enums_visible): New constant.  This ports to
GCC 3.4.2 the export of symbols to GDB.  Problem reported by Eli
Zaretskii in <http://bugs.gnu.org/12036#13>.
* lisp.h (PUBLISH_TO_GDB): Remove.  All uses removed.  No longer
needed now that we have gdb_make_enums_visible.
(enum CHECK_LISP_OBJECT_TYPE, enum Lisp_Bits, enum More_Lisp_Bits)
(enum enum_USE_LSB_TAG):
New enum types, packaging up enums that need to be exported to GDB.