Clang support already existed in the Makefiles but was not complete
and some gcc options did not work with clang. Move those to be conditional
on the compiler used to make clang work.
To build with clang for x86:
make CC=clang -C samples/microkernel/apps/hello_world/
You still need the gcc cross environment for various tools.
For now, only x86 was tested.
Change-Id: I1a50c3a82d79ff3001beb4366961ca810eeb6006
Signed-off-by: Anas Nashif <anas.nashif@intel.com>