Don't try to push GCC flags to the LLVM-based toolchain

If (or as) RIOT expects to have interoperability between object files from different compilers, the CFLAGS passed to LLVM should be equivalent, interoperability-wise, to those to GCC.

The RIOT_CFLAGS extraction process could be changed to not use the CFLAGS as used for whichever compiler is just in use, but to use the CFLAGS as they would be passed to LLVM unconditionally.

This probably needs a bit of digging in the makefiles, as RIOT probably doesn't even include the other toolchain's files when running for GCC.