Don't define __ASSUME_UTIMES for linux-generic architectures.
The __ASSUME_UTIMES macro describes whether the utimes syscall is
present. For linux-generic architectures, it isn't (utimensat is
instead), so the macro should not be defined for them; this patch
removes the spurious definitions for such architectures. (Those
definitions don't actually cause any user-visible bug, because
futimes.c doesn't use __ASSUME_UTIMES if __ASSUME_UTIMENSAT is
defined, and futimesat.c and utimes.c are overridden for
linux-generic, but the definitions are still logically incorrect.)