linux: Fix time64 support for futimesat

The generic implementation does not support time64 and the default
one return overflow for invalid tv_sec with UTIME_NOW / UTIME_OMIT
(which is valid since tv_sec in such cases is ignored by the
kernel).

Checked on x86_64-linux-gnu and i686-linux-gnu (on 5.4 and on 4.15
kernel).

Reviewed-by: Lukasz Majewski <lukma@denx.de>