[CMake] If you're not installing the libcxx library, exclude it from the "all" target so it doesn't get built when you run "ninja install"

This is just a build dependency optimization. Running check-libcxx will still build libcxx and function as expected, it just removes libcxx from the all build and install targets.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@254628 91177308-0d34-0410-b5e6-96231b3b80d8
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