Add placeholder _sysconfigdata_* files

This lets sysconfig.get_config_vars() return successfully, even if it
has fewer values than normal installs. This lets a number of other
functions work reasonably without throwing exceptions
(sysconfig.get_path, sysconfig.get_platform on Mac)

Bug: 176214252
Bug: 197408991
Change-Id: I8a4b5713d3344d84480bad087e230f07ea6253d4