[swift-3.0-preview-2-branch] build-script fixes (#3234)
* build-script: unbreak the 'build-swift-stdlib' option
Argparse was trying to helpfully match it with
'build-swift-stdlib-unittest-extra', but this wasn't our intention.
rdar://problem/27030803
* [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
