swift-package-manager

[POSIX] Reexport TOOLCHAINS when building.

 - This is very convenient when used with xcrun's indirection on OS X, it means
   you can do this:

     env TOOLCHAINS=swift Utilities/bootstrap

   and the build will automatically run against the latest toolchain (no PATH
   munging required).

   This depends a tiny bit on features from Xcode 7.3 beta, but once that GMs we
   should update the docs to make this the standard development-from-snapshot
   build instructions.