[lldb] Move once_flags in HostInfoLinux so the internal state struct

The HostInfoLinuxFields struct is supposed to be set up/torn down on
Initialize/Terminate and should contain all the state of the plugin.
`once_flags` are part of this state and should also be reset on `Terminate` so
we can re-initialize these lazy values after the next `Initialize` call.

This itself is NFC as the HostInfoLinux was broken before this patch and is
still broken afterwards. D104091 will be the proper fix.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D104093