Daniel Schürmann authored Nov 01, 2022 and Brad King committed Nov 01, 2022

Prior to commit a9073db7 (IAR: update language specification
detection, 2021-09-23, v3.22.0-rc1~83^2) 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