Updated unit test engine to set default build platform based on OS.

Summary: This updates the unit test engine to use `phutil_is_windows()` to determine whether it should by default create Windows or Linux projects when the user has not yet generate projects.

Test Plan: Use `git reset --hard HEAD && git clean -xdf` to clean out any projects you may have.  On Windows running `arc unit --everything` should cause it to generate Windows projects.

Reviewers: Sabre070

Reviewed By: Sabre070

Differential Revision: http://code.redpointsoftware.com.au/D35