buildroot

gcc: allow the selection of upstream GCC 7.x for ARC

Upstream gcc now has support for ARC, as of 7.x, with 7.2 being
usable, so let's allow its selection.

GCC from Synopsys GitHub still has a lot of fixes and improvements
most of which will be a part of the next upstream release but let's
see how it goes and if we see vanilla GCC doesn't require way too many
back-ported patches we'll be able to get rid of Synopsys GitHub here
at all.

As of today the only really required back-port is the one for non-uClibc
libc.

Back-ported from upstream master with help of:
-------------------->8--------------------
git format-patch 927c0132251f -1 -- . ':!gcc/ChangeLog' ':!libgcc/ChangeLog'
-------------------->8--------------------

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>