toolchain: bump ARC toolchain components to arc-2015.12-rc1

toolchain: bump ARC toolchain components to arc-2015.12-rc1

This change introduces newer ARC toolchain in Buildroot.
Note this is the first release candidate and we'll probably see another
RC before cutting the final release.

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 updated to v4.8.5
 * GDB updated to 7.10

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-rc1

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