What's in this pull request?

It's possible to cross-compile for iOS while skipping stdlib tests on OS X:

$ utils/build-script --ios -- --skip-test-osx

The same is not possible on Linux. Add options to skip builds and tests on non-Darwin platforms: Linux, FreeBSD, and Cygwin. This makes the following possible:

$ utils/build-script --some-other-platform --skip-test-linux

Why merge this pull request?

• This ports functionality that exists for OS X to Linux, FreeBSD, and Cygwin.
• Although none of these platforms are currently capable of cross-compiling, the Android fork cross-compiles from Linux to Android. These options would immensely useful there, since otherwise there is no easy way to only run the tests for Android when building on Linux.

What downsides are there to merging this pull request?

• One could argue that it should be merged alongside the Android fork. However, that would conflate that set of changes (a port of Swift to a new platform) with these (more options for the build script).