ARC: scripts: twister: fix toolchain filtering

Let's not provide default toolchains for 'arc' intentionally: some
targets can't be built with GNU toolchain ("zephyr", "cross-compile",
"xtools" options) and for some targets we haven't provided MWDT
compiler / linker options in corresponding SoC file in Zephyr, so
these targets can't be built with ARC MWDT toolchain
("arcmwdt" option) by Zephyr build system.
Instead for 'arc' we rely on 'toolchain' option in board yaml
configuration.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>