linux: Disable fstatat64 fallback if __ASSUME_STATX is defined

If the minimum kernel supports statx there is no need to call the
fallback stat legacy syscalls.

The statx is also called on compat xstat syscall, but different
than the fstatat it calls no fallback and it is assumed to be
always present.

Checked on powerpc-linux-gnu (with and without --enable-kernel=4.11)
and on powerpc64-linux-gnu.