ARC: rework setting of ARC CPU specific compiler options

It's a very rare if at all existing occasion when ARC CPU template
is used as is w/o any changes - in the end it's a beauty and competitive
advantage of ARC cores to be tailored for a particular use-case - and
so it doesn't make a lot of sense to offer template-based "-mcpu" selection.
Given for each and every platform we end-up adding quite a few more flags
it's logical to move "-mcpu" selection to platform's definition as well
which we exactly do here.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>