runtime: disable dynamic priority boosting on windows
Windows dynamic priority boosting assumes that a process has different types
of dedicated threads -- GUI, IO, computational, etc. Go processes use
equivalent threads that all do a mix of GUI, IO, computations, etc.
In such context dynamic priority boosting does nothing but harm, so turn it off.
In particular, if 2 goroutines do heavy IO on a server uniprocessor machine,
windows rejects to schedule timer thread for 2+ seconds when priority boosting is enabled.
Fixes issue 5971.

R=alex.brainman
CC=golang-dev
https://codereview.appspot.com/12406043