[lldb] Creates _liblldb symlink from cmake
This is another attempt of D67993.
This change removed hard coded relative paths. This way we can generate correct result when get_python_lib() returns a different path, or LLDB_PYTHON_RELATIVE_PATH is specified directly.
By moving things out of python, we are also able to correctly process more cross compile situations. E.g. .pyd vs .so for Windows.
Subscribers: lldb-commits, mgorny
Differential Revision: https://reviews.llvm.org/D68858