Add _LP64 and __LP64__ for LP64 programming model.

[H.J.]

Hi,

All the current x86-64 psABI compilers define _LP64 and __LP64__.
GCC started to define them in 2003:

http://gcc.gnu.org/ml/gcc-patches/2002-10/msg00692.html
http://gcc.gnu.org/ml/gcc-patches/2003-03/msg00780.html

But they aren't documented in the psABI. X32 GCC doesn't define
them for obvious reason. I added them to x32 psABI for LP64
programming
model.

H.J.