[lldb] Move PlatformPOSIX::ConnectToWaitingProcesses to RemoteAwarePlatform

The functionality is not posix specific. Also force the usage of the
gdb-remote process plugin in the gdb platform class.

This is not sufficient to make TestPlatformConnect pass on windows (it
seems it suffers from module loading issues, unrelated to this test),
but it at least makes it shut down correctly, so I change the skip to an
xfail.