x32: Check __ILP32__ instead of __LP64__ for x32

Check __LP64__ isn't a reliable way to tell if we are compiling for x32
since __LP64__ isnn't specified by x86-64 psABI.  Not all x86-64
compilers define __LP64__, which was added to GCC 3.3. The updated x32
psABI:

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



definse _ILP32 and __ILP32__ for x32.  GCC trunk and 4.7 branch have
been updated to define _ILP32 and __ILP32__ for x32.  This patch
replaces __LP64__ check with __ILP32__.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>