Prefer gnu-stat if available

Busybox stat is not enough but is we had a working alternatives, the default
stat command should be gnu stat as in older versions of Turris OS. So if there
is a gnu-stat, use that one as it is bound to work, if not, let's assume the
stat is linking to the correct implementation (dependencies are in package) and
we are running either in test environment or on old/new Turris OS version.