Use explicit add_subdirectory's for LLVM target sublibraries instead

Use explicit add_subdirectory's for LLVM target sublibraries instead
of testing for its presence at cmake time.

This way the build automatically regenerates the makefiles when a svn
update brings in a new sublibrary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@126068 91177308-0d34-0410-b5e6-96231b3b80d8