uclibc: get rid of ARC's uClibc fork

For quite some time all ARC changes have been pushed upstream.
So both uClibc master branch and uClibc-ng have full ARC support
including the most recent changes.

I.e. finally we have reached the point when there's no need in
Synopsys fork of uClibc on GitHub and it will soon be obsoleted and
removed at some pint of time.

Now since vanilla uClibc hasn't seen any releases for many years we cannot
really use it for ARC. Fortunately there exists uClibc-ng and Waldemar
cuts releases regularly - that works perfectly fine for us.

That patch removes all mentions of ARC's uClibc which in turn makes
uClibc-ng default libc for ARC.

Cc: Waldemar Brodkorb <wbx@openadk.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-By: Waldemar Brodkorb <wbx@uclibc-ng.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>