Make logging work with new runtime code

[brson]

Whatever upcall or lang item rustc is calling for log needs to be updated to figure out exactly how to emit that log. There are several scenarios:

1. A thread-local rust_task* is available - use the current path
2. A thread-local Scheduler is available - use some new scheduler-local mechanism
3. A newsched Task is running - use some new task-local mechanism
4. There is no available execution context - maybe these should just printf to the console. not sure

Consider redesigning log in the process #3309

[emberian]

Visiting for triage; carry on.

[alexcrichton]

@brson, after glancing at the logging code, this seems done now, but I just wanted to make sure.