toolchains: combine host variants

Combine toolchains provided by the host into one variant. This includes
both gcc and llvm installationt typically coming from the distribution
(on Linux).

Both gcc and llvm are now part of the 'host' variant, the default is the
gnu compiler, so setting

	ZEPHYR_TOOLCHAIN_VARIANT=host

Will select the gnu compiler. To select llvm or any other compuler
provided in this variant, use the follwoing format:

	ZEPHYR_TOOLCHAIN_VARIANT=<variant>/<compiler>

The following will select llvm:

	ZEPHYR_TOOLCHAIN_VARIANT=host/llvm

Although gnu is the default, it can also be selected using the above
syntax:

	ZEPHYR_TOOLCHAIN_VARIANT=host/gcc

This commit removes the llvm variant for now, it should be deperecated
in another commit to follow.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>