[lldb] add fallback for LLDB_PYTHON_RELATIVE_PATH

Some pythons are configured to set platlib somewhere outside of their
sys.prefix.   It's important that we at least use some reasonable
default for LLDB_PYTHON_RELATIVE_PATH even in that case, because
even if the user overrides it on the cmake invocation, cmake will
still be called without the override in order to build tablegen.

Reviewed By: JDevlieghere, clayborg

Differential Revision: https://reviews.llvm.org/D114973