[libc] Add backwards-compatibility macro to struct stat. (#182601)
Older code may use `st_atime` which recorded timestamps with one-second
precision, instead of `struct timespec st_atim` that is available in
later POSIX versions.
Add `#define st_atime` (& friends) to type declaration as suggested in
https://man7.org/linux/man-pages/man3/stat.3type.html