cmake: llvm: Make the setup for host installed clang easier

The logic is practically intact and is the following:

1. Use any host installed llvm/clang in the path in case
   ZEPHYR_TOOLCHAIN_VARIANT=llvm is requested alone.
2. This can be further restricted with TOOLCHAIN_HOME.
3. And can be further overridden with CLANG_ROOT_DIR,
   like previously.

So, only the unconditional restriction to /usr is lifted.

Together with fixing the unconditional set of TOOLCHAIN_HOME
by host tools for non-toolchain needs, this makes the logic
more flexible.

Now, after the logic is controllable by TOOLCHAIN_HOME, 3)
might be an extra, but is left intact for backward compatibility.

Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>