IAR: Restore support for CMAKE_CXX_FLAGS_INIT
Prior to !6458 (merged) the --c++/--eec++ flags
were stored in CMAKE_IAR_CXX_FLAG so that they would be used as part
of the CMAKE_CXX_COMPILE_OBJECT rule variable. That commit moved the
flags to CMAKE_CXX_FLAGS, which also broke initialization of
CMAKE_CXX_FLAGS from CMAKE_CXX_FLAGS_INIT. Restore the original
approach.
Fixes: #24111 (closed)
Backport: release
Edited by Brad King