Add support for compiling with clang on x86/linux

Add `Config.x86_linux_clang` to use clang instead of gcc to build
cmucl.  update `create-target.sh` so that it sets the motif variant
correctly when using `Config.x86_linux_clang`

See issue #68.

With this config, Fedora 28 successfully builds cmucl and passes all
the tests.