tests: fix *stat64 tests on alpha

tests: fix *stat64 tests on alpha

On some architectures including alpha, <asm/stat.h> provides a
definition of struct stat that has no st_atime_nsec, st_mtime_nsec, and
st_ctime_nsec fields.  At the same time, struct stat64 always has these
fields.  Fix tests to take this difference into account.

* tests/fstat64.c (STRUCT_STAT_IS_STAT64): New macro, defined to 1.
* tests/lstat64.c (STRUCT_STAT_IS_STAT64): Likewise.
* tests/stat64.c (STRUCT_STAT_IS_STAT64): Likewise.
* tests/xstatx.c [!STRUCT_STAT] (STRUCT_STAT_IS_STAT64): New macro,
defined to 0.
[USE_ASM_STAT && STRUCT_STAT_IS_STAT64]:
(HAVE_STRUCT_STAT_ST_ATIME_NSEC, HAVE_STRUCT_STAT_ST_CTIME_NSEC,
HAVE_STRUCT_STAT_ST_MTIME_NSEC): Redefine to 1.