Allow customized relative PYTHONHOME

Summary:
This change allows a hard coded relative PYTHONHOME setting. So that
python can easily be packaged together with lldb.

The change includes:
1. Extend LLDB_RELOCATABLE_PYTHON to all platforms. It defaults to ON
for platforms other than Windows, to keep the behavior compatible.
2. Allows to customize LLDB_PYTHON_HOME. But still defaults to
PYTHON_HOME.
3. LLDB_PYTHON_HOME can be a path relative to liblldb. If it is
relative, we will resolve it before send it to Py_DecodeLocale.

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74727