testsuite: interpose the large-file spellings of open/openat/fstatat

[samueloph]

Which symbol names this test's LD_PRELOAD hook exports is decided by the compiler that builds it, and which names rsync imports is decided by configure -- and the two do not have to agree.

Where off_t is not already 64 bits, configure's AC_SYS_LARGEFILE adds -D_FILE_OFFSET_BITS=64 (i386 and alpha in Debian), so glibc redirects every open()/openat()/fstatat() call in rsync to open64(), openat64() and fstatat64(). The hook is compiled by a bare "cc", so on those architectures it defines only the unsuffixed names: the receiver's opens never reach it, no EACCES is injected, the marker the positive control looks for is never written, and the test fails with

positive control failed: receiver did not open the existing partial file
with O_CREAT (rc=0, output='')

It happens to work on Debian's 64-bit time_t ports (armhf, hppa, powerpc, ...) only by luck: their gcc predefines -D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64, so glibc's __REDIRECT renames the hook's own DEFINITIONS as well and it ends up exporting exactly the *64 names rsync imports.

Define both spellings explicitly so the hook interposes whichever set the rsync under test was linked against, and #undef the two macros at the top of the hook so that renaming cannot happen -- otherwise, on precisely those ports whose compiler predefines them, open() would be emitted as open64() and collide with the explicit wrapper ("symbol `open64' is already defined"), leaving the hook unbuildable and the test skipped.

The new pointers are resolved through hook_resolve(), so the existing nested-dlsym recursion guard covers them as well.

Failing build logs:
i386 https://buildd.debian.org/status/fetch.php?pkg=rsync&arch=i386&ver=3.5.0%2Bds1-1&stamp=1786938290&raw=0
alpha https://buildd.debian.org/status/fetch.php?pkg=rsync&arch=alpha&ver=3.5.0%2Bds1-1&stamp=1786973432&raw=0

[steadytao]

LGTM. Thank you.