swift234

[CMake] Only build StdlibUnittest when SDK overlay is built

On Linux it used to be possible to build only the stdlib, without the
SDK overlays, like so:

```
utils/build-script -- --build-swift-stdlib --build-swift-sdk-overlay=0
```

However this invocation now results in the following error:

```
+ /usr/bin/cmake --build /home/modocache/GitHub/apple/build/Ninja-ReleaseAssert/swift-linux-x86_64 -- -j8 all swift-stdlib-linux-x86_64
ninja: error: '/home/modocache/GitHub/apple/swift/stdlib/private/SwiftPrivatePthreadExtras/swiftGlibc-linux-x86_64', needed by 'stdlib/private/SwiftPrivatePthreadExtras/linux/x86_64/SwiftPrivatePthreadExtras.o', missing and no known rule to make it
utils/build-script: fatal error: command terminated with a non-zero exit status 1, aborting
```

The problem is that SwiftPrivatePthreadExtras is always built, regardless
of whether the SDK overlay is built. I believe there's an explicit check
against this for Darwin platforms to prevent the same error.

The solution, implemented here, is to add the same check for Linux.