remove LFS64 programming interfaces (macro-only) from _GNU_SOURCE

these badly pollute the namespace with macros whenever _GNU_SOURCE is
defined, which is always the case with g++, and especially tends to
interfere with C++ constructs.

as our implementation of these was macro-only, their removal cannot
affect any existing binaries. at the source level, portable software
should be prepared for them not to exist.

for now, they are left in place with explicit _LARGEFILE64_SOURCE.
this provides an easy temporary path for integrators/distributions to
get packages building again right away if they break while working on
a proper, upstreamable fix. the intent is that this be a very
short-term measure and that the macros be removed entirely in the next
release cycle.

include/aio.h

@@ -49,7 +49,7 @@ int aio_fsync(int, struct aiocb *);
 
 int lio_listio(int, struct aiocb *__restrict const *__restrict, int, struct sigevent *__restrict);
 
-#if defined(_LARGEFILE64_SOURCE) || defined(_GNU_SOURCE)
+#if defined(_LARGEFILE64_SOURCE)
 #define aiocb64 aiocb
 #define aio_read64 aio_read
 #define aio_write64 aio_write