Do not use 'u' flag for ar

Do not use 'u' flag for ar

The Reproducible Builds effort (https://wiki.debian.org/ReproducibleBuilds) in Debian (at least) means that 'ar' is built in deterministic mode as default: all timestamps are set to 0.

This is not compatible with the use of the 'u' flag, and therefore ARFLAGS has to be changed from 'cru' to 'cr'.

This gets rid of the harmless but annoying warning
  ar: `u' modifier ignored since `D' is the default (see `U')
(cherry picked from commit 987fc1d15d017c192a566fc922e3967d9d280668)