toolchain: bump ARC toolchain components to arc-2015.12

This change introduces newer ARC toolchain in Buildroot.

That new arc-2015.12 release doesn't bring any significant changes
but mostly is focused on fixes and minor improvements here and there.
Most noticeable changes are:
 * GCC: Source update to v4.8.5
 * GDB: Updated to upstream 7.10 release.

You may find more info on fixes and improvements in that release at:
https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/releases/tag/arc-2015.12



Signed-off-by: Lada Trimasova <ltrimas@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>