toolchain: Bump ARC tools to arc-2018.09-rc2

toolchain: Bump ARC tools to arc-2018.09-rc2

This commit bumps ARC toolchain to arc-2018.09-rc2,
which includes significant changes since arc-2018.09-rc1.
We want to test how new toolchain-rc2 builds packages,
so we can make fixes before release of toolcain.
This makes us closer to toolchain release which will be in a few weeks.

Please note that it is a release candidate and it might
contain some breakages, please don't use it for production builds.

Signed-off-by: Evgeniy Didin <Evgeniy.Didin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: arc-buildroot@synopsys.com
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>