swift234

[build-script] Support concatenated stdlib-deployment-targets.

 - This fixes a regression introduced in
   a4537e8a0fba765343e1b62af243055fc922f18d, where we used to accept a list of
   concatenated deployment targets as a single argument.

 - <rdar://problem/26928189> build-script fails to split deployment targets
   before iterating over them