arc: Use "-mcpu=archs" instead of deprecated "-marchs" for ARC HS

Newer ARC toolchains don't support "-marchs" option any longer.
Instead "-mcpu=archs" should be used. What's also important older
toiolchains that support ARC HS cores will also happily accept
"-mcpu=archs" so that's a very safe move.

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