Fix compilation flags
- fix setting of architecture flags for Clang
- define CMake compiler id and version variables in the toolchain
- CMake defines them too late to be able to use them in the toolchain, but it's practical to write code conditional to teh compiler version using standard variables instead of custom ones
- add
-Wsuggest-override
to clang >= 11.0 (and fix the logic to useCMAKE_CXX_COMPILER_ID
instead ofLCG_COMPILER
)