gcc: arc: bump to arc-4.8-R3 release

gcc: arc: bump to arc-4.8-R3 release

This patch fixes several internal compiler errors including:
http://autobuild.buildroot.net/results/2858bb1ca1d12955c837da5a8b7d69c587a75646/
http://autobuild.buildroot.net/results/85deef67654019a630d6cbc37f3dbbbc071aad38/

Support for the GCC built-in atomic functions is also added and is enabled
with option -matomic. Atomic built-ins require LLOCK/SCOND instructions to
be enabled in the ARC 700 CPU.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>