cmake: add metaware toolchain support in build system

* add metaware toolchain support in build system:
   * compiler
   * linker
   * binutils
     * gcc objcopy is used because it can't be replaced
       with mwdt's binutils currently
* To use ARC metaware toolchain, you'd better:
   * in Linux/Unix environment
   * install arc gcc/zephyr toolchain to use gnu's objcopy
     tool
   * set ZEPHYR_TOOLCHAIN_VARIANT=arcmwdt

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>
Signed-off-by: Watson Zeng <zhiwei@synopsys.com>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>