xstat: only check to see if __ASSUME_ST_INO_64_BIT is defined

We define __ASSUME_ST_INO_64_BIT by default for Linux targets, and then
undef it for alpha/sh targets.  But the code that uses it looks at its
value (as 0/1) rather than whether it's defined (like all other assume
knobs).  Change the code to see if it's defined to fix build Wundef build
errors for alpha/sh.