[libcxxabi] Fix cmake order dependency wrt dllexporting

If LIBCXX_ENABLE_SHARED isn't explicitly set on the cmake command
line, isn't set in the cache, and the libcxxabi project is configured
before libcxx, then LIBCXX_ENABLE_SHARED isn't defined yet. Once
the libcxx cmake project has been parsed, LIBCXX_ENABLE_SHARED would
have been set to its default value of ON.

This makes sure that the symbols are properly dllexported in such
a configuration scenario.

Differential Revision: https://reviews.llvm.org/D120982