cmake: allow to specify compliler for DTS preprocessing

MWDT CCAC compiler can't handle the dts file as gcc does, so
allow to specify compliler for DTS preprocessing which is differ
from ${CMAKE_C_COMPILER}.

Use ARC GNU compiler for DTS preprocessing in case of MWDT
toolchain.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
2 files changed