PATCH: Cast to int32 first when casting pointer to int64

[Roland McGrath]

> * 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.

libc-internal.h is a catch-all for when nobody can think of a better
topical header to use (or invent).  If throwing too many things into
libc-internal.h causes problems, then things should be split out into
smaller headers with meaningful names and groupings.