PATCH: Cast to int32 first when casting pointer to int64

[Florian Weimer]

* Roland McGrath:

> These are new private macros.  They don't really belong in any shadow
> version of an installed header.  Put them in some purely internal header.
> If there isn't an obvious good choice, libc-internal.h is probably OK.

The problem with this approach is that <libc-internal.h> is now
included everyone on x86_64, causing things to build which break on
most other architectures.

Can we make the macro definition x86_64-only?  It is used for i386 as
well, but it's not clear if it's not obscuring real bugs there.