Improve accuracy of time measurements on Win32

The Microsoft .Net implementation of the Process calls provides
low-resolution timers. Using System.Diagnostics.Stopwatch provides
high-resolution timers both there and on Mono.

Also, try to pin the process to a specific CPU and to increase the
priority, in order to reduce noise while benchmarking.
Make fsharp compiler name an env property

On Unix the default name of the fsharp compiler is "fsharpc", while
Visual Studio installs it as fsc. Abstracting it in a variable makes
it possible to compile on Windows with "make FSC=fsc" (assuming you
have a working "make" command and fsc is in the path).