linux: Consolidate utimes

The generic version does not have time64 support and Linux default
uses utimensat.  With hppa version gone, __ASSUME_UTIMES is not used
anymore.

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

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>