Add an LLVM backend and a clang toolchain variant to support building
with llvm coming with popular Linux distributions.
This has been tested with X86 boards:
- quark_d2000_crb
- quark_se_c1000_devboard/Arduino 101
Use:
export ZEPHYR_TOOLCHAIN_VARIANT=clang
Signed-off-by: Anas Nashif <anas.nashif@intel.com>