llvm: don't do out-of-tree builds manually

cmake.bbclass as of oe-core 783fb88 defaults to out-of-tree builds.  Use that
functionality directly to clean up the recipe.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>