[libfuzzer] No Large File Support in 32bit targets

[ernst-schwaiger]

FuzzerIOPosix.cpp hasn't enabled Large File Support/LFS for 32bit targets. All stat() invocations return EOVERFLOW on large file systems. Consequently, IsFile(), IsDirectory(), FileSize() will always return false/zero and e.g. my32BitFuzzBinary CORPUS stops with "ERROR: The required directory "CORPUS" does not exist"

Patching FuzzerIOPosix.cpp like this overcomes the issue (albeit adding -D_FILE_OFFSET_BITS=64 to the CMakefiles is a cleaner solution):

//...
#if (LIBFUZZER_POSIX || LIBFUZZER_FUCHSIA)

// turn LFS on for i386 (and other supported 32 bit platforms)
#if __i386
  #define _FILE_OFFSET_BITS 64
#endif

#include "FuzzerExtFunctions.h"
#include "FuzzerIO.h"
#include <cstdarg>
#include <cstdio>
#include <dirent.h>
#include <fstream>
#include <iterator>
#include <libgen.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

// ensure LFS is turned on
static_assert(sizeof(off_t) == 8, "Large file interface is not present, see _FILE_OFFSET_BITS in https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html");

namespace fuzzer {
//...