Add a command-line flag to set the target platform

[gvanrossum]

This is a follow-up task from #698.

This would allow us to type-check code meant for Windows on a Mac, for example. It would control the value assumed by sys.platform. The value should probably be limited to a few values, e.g. darwin for Mac, win32 for Windows, 'linux2' for Linux, and 'freebsd' for, well, Free BSD. (There are a bunch of other possible values too, and maybe we should collect a complete list, but I want to make sure people don't accidentally try to set it to e.g. mac or osx or windows or nt or even plain linux.)