build: add -D_FILE_OFFSET_BITS=64 to _CPPFLAGS instead of _CFLAGS

build: add -D_FILE_OFFSET_BITS=64 to _CPPFLAGS instead of _CFLAGS

* tests/Makefile.am (fstat64_CFLAGS): Rename to fstat64_CPPFLAGS,
replace $(AM_CFLAGS) with $(AM_CPPFLAGS).
(fstatat64_CFLAGS): Rename to fstatat64_CPPFLAGS,
replace $(AM_CFLAGS) with $(AM_CPPFLAGS).
(ftruncate64_CFLAGS): Rename to ftruncate64_CPPFLAGS,
replace $(AM_CFLAGS) with $(AM_CPPFLAGS).
(lstat64_CFLAGS): Rename to lstat64_CPPFLAGS,
replace $(AM_CFLAGS) with $(AM_CPPFLAGS).
(mmap64_CFLAGS): Rename to mmap64_CPPFLAGS,
replace $(AM_CFLAGS) with $(AM_CPPFLAGS).
(newfstatat_CFLAGS): Rename to newfstatat_CPPFLAGS,
replace $(AM_CFLAGS) with $(AM_CPPFLAGS).
(stat64_CFLAGS): Rename to stat64_CPPFLAGS,
replace $(AM_CFLAGS) with $(AM_CPPFLAGS).
(statfs_CFLAGS): Rename to statfs_CPPFLAGS,
replace $(AM_CFLAGS) with $(AM_CPPFLAGS).
(truncate64_CFLAGS): Rename to truncate64_CPPFLAGS,
replace $(AM_CFLAGS) with $(AM_CPPFLAGS).
(uio_CFLAGS): Rename to uio_CPPFLAGS,
replace $(AM_CFLAGS) with $(AM_CPPFLAGS).