toolchain/arcmwdt: Support SDK 0.18 directory layout

The arcmwdt toolchain uses some tools from the Zephyr SDK. With version
0.18, the path to the GNU generic.cmake file has moved. Adapt to that by
checking for the new location and using that if available.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

cmake/toolchain/arcmwdt/generic.cmake

@@ -30,7 +30,11 @@ set(TOOLCHAIN_HOME ${ZEPHYR_SDK_INSTALL_DIR})
 # but in hew HWMv2 model ARCH variable will be initialized more later.
 # This workaround uses any (first awailable, independent on ARCH) toolchain from SDK for DTC preprocessing only.
 # For other build stages ARCMWDT will be used.
-include(${ZEPHYR_SDK_INSTALL_DIR}/cmake/zephyr/generic.cmake)
+if(EXISTS ${ZEPHYR_SDK_INSTALL_DIR}/cmake/zephyr/gnu/generic.cmake)
+  include(${ZEPHYR_SDK_INSTALL_DIR}/cmake/zephyr/gnu/generic.cmake)
+else()
+  include(${ZEPHYR_SDK_INSTALL_DIR}/cmake/zephyr/generic.cmake)
+endif()
 unset(TOOLCHAIN_HAS_NEWLIB CACHE)
 unset(TOOLCHAIN_HAS_PICOLIBC CACHE)