Update HP_TIMING_NOW for _ISOMAC in sysdeps/generic/hp-timing.h

commit e9698175
Author: Lukasz Majewski <lukma@denx.de>
Date:   Mon Mar 16 08:31:41 2020 +0100

    y2038: Replace __clock_gettime with __clock_gettime64

breaks benchtests with sysdeps/generic/hp-timing.h:

In file included from ./bench-timing.h:23,
                 from ./bench-skeleton.c:25,
                 from
/export/build/gnu/tools-build/glibc-gitlab/build-x86_64-linux/benchtests/bench-rint.c:45:
./bench-skeleton.c: In function ‘main’:
../sysdeps/generic/hp-timing.h:37:23: error: storage size of ‘tv’ isn’t known
   37 |   struct __timespec64 tv;      \
      |                       ^~

Define HP_TIMING_NOW with clock_gettime in sysdeps/generic/hp-timing.h
if _ISOMAC is defined.  Don't define __clock_gettime in bench-timing.h
since it is no longer needed.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>