[winrt support] Adding a new feature to help support building for the Windows Runtime

Changes to enable building for the Windows Runtime (store/phone). Separated out from previous pull request, https://github.com/boostorg/build/pull/9. The changes for running tests for the Windows Runtime from the previous pull request will come in a separate change once this one is done.

1. Adds a new feature <windows-api> for specifying the WINAPI_FAMILY API set to target. Defaults to desktop, which is how builds before my changes.
2. A few small changes for adding ARM support.
3. Changes to call to correct vcvars setup script for Windows Phone 8.