checksyscalls: Unconditionally ignore fstat{,at}64

These can be replaced by statx().  Since rv32 has a 64-bit time_t we
just never ended up with them in the first place.  This is now an error
due to -Werror.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>