[Backport] Cache PYTHON_EXECUTABLE for windows

Summary: This way it can be overwritten when cross compiling.

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D67641

Change-Id: I4f1affc21bcc43bcb437e05c8d7f876fd207ce48
llvm-svn: 372194