linux: use statx for fstat if neither newfstatat nor fstatat64 is present

LoongArch is going to be the first architecture supported by Linux that
has neither fstat* nor newfstatat [1], instead exclusively relying on
statx. So in fstatat64's implementation, we need to also enable statx
usage if neither fstatat64 nor newfstatat is present, to prepare for
this new case of kernel ABI.

[1]: https://lore.kernel.org/all/20220518092619.1269111-1-chenhuacai@loongson.cn/



Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>