IAR defaults to C99 mode, but doesn't enable VLAs by default. Enable them
to make it more conformant.
We don't have much if any code using actual variable-length arrays, but
variably-modified types are occasionally used. The same switch controls
both.
(VLAs were actually already enabled in most of the project export
templates, but not the build script).
- vla flag is not compatible with c++ (not supported), it generates an error
in the IDE. Therefore we remove it
- common flags - add dlib and thum to the common flags.
- cpu flag is for only runtime cmd, IDE sets it via defined MCU, not required.