Add __ILP32 and __ILP32__ for X32 programming model

[H.J. Lu]

Hi,

We need a reliable way to tell if we are compiling for x32 through
pre-defined preprocessor symbol.  __LP64/__LP64__ aren't
specified by x86-64 psABI, although they have been added to
GCC 3.3.  They can't be counted on to detect x32 since not x86-64
compilers define them.   I updated x32 psABI:

https://sites.google.com/site/x32abi/documents

to define __ILP32 and __ILP32__ for X32 programming model.  I
will submit a patch for GCC trunk and 4.7 branch.

Thanks.

-- 
H.J.