struct stat is not posix conformant on microblaze with __USE_FILE_OFFSET64

Commit a06b40cd updated stat.h to use
__USE_XOPEN2K8 instead of __USE_MISC to add the st_atim, st_mtim and
st_ctim members to struct stat. However, for microblaze, there are two
definitions of struct stat, depending on the __USE_FILE_OFFSET64 macro.
The second one was not updated.

Change __USE_MISC to __USE_XOPEN2K8 in the __USE_FILE_OFFSET64 version
of struct stat for microblaze.