[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.

Author: modocache

stdlib/private/CMakeLists.txt

@@ -2,23 +2,16 @@ if(SWIFT_BUILD_STDLIB OR SWIFT_BUILD_SDK_OVERLAY)
   add_subdirectory(SwiftPrivate)
 endif()
 
-if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
-  if(SWIFT_BUILD_SDK_OVERLAY)
-    # FIXME: there is nothing Darwin-specific in StdlibUnittest, but to use
-    # POSIX APIs it imports the Darwin module on Apple platforms, so it can't
-    # be built separately from the SDK overlay.
-    add_subdirectory(StdlibUnittest)
-    add_subdirectory(StdlibCollectionUnittest)
-    add_subdirectory(StdlibUnittestFoundationExtras)
-    add_subdirectory(SwiftPrivateLibcExtras)
-    add_subdirectory(SwiftPrivatePthreadExtras)
-    add_subdirectory(SwiftReflectionTest)
-  endif()
-endif()
-
-if(CMAKE_SYSTEM_NAME STREQUAL "Linux" OR CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")
+if(SWIFT_BUILD_SDK_OVERLAY)
+  # SwiftPrivatePthreadExtras makes use of Darwin/Glibc, which is part of the
+  # SDK overlay. It can't be built separately from the SDK overlay.
   add_subdirectory(StdlibUnittest)
   add_subdirectory(StdlibCollectionUnittest)
   add_subdirectory(SwiftPrivateLibcExtras)
   add_subdirectory(SwiftPrivatePthreadExtras)
+
+  if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+    add_subdirectory(StdlibUnittestFoundationExtras)
+    add_subdirectory(SwiftReflectionTest)
+  endif()
 endif()