ARCMWDT: Fix building under hwmv2 model

On very early stage build system needs to get access to DTC preprocessor.
MWDT has no it's own preprocessor, so here zephyr-SDK preprocessor is used.
On latest build stages zephyr-SDK objcoby also required as MWDT binutils
don't support all features. This at the same time requires that
ZEPHYR_SDK_INSTALL_DIR must be initialized with valid arch-dependent
prefix.

Zephyr-SDK requires ARCH variable to be initialized before
include "generic.cmake", but in hew HWMv2 model ARCH variable will be
initialized more later. This workaround uses any (first awailable,
independent on ARCH) toolchain from SDK for DTC preprocessing only.
For other build stages ARCMWDT will be used.

Signed-off-by: Nikolay Agishev <agishev@synopsys.com>