Non-Darwin SDKs product results in per-arch directories.

Depends on swiftlang/swift#19432

Unix (other than Darwin) and Windows do not support fat binaries.
However, the build system was placing the build results in the
platform/OS folder, instead of the per-architecture folder. Having two
architectures side by side was impossible.

After applying the above patch in the Swift compiler, non-Darwin
platforms will look into the per-architecture folders, so the sibling
projects need to leave the products in the right place.

Author: drodriguez

CMakeLists.txt

@@ -148,4 +148,9 @@ install(FILES
           ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_SHARED_LIBRARY_PREFIX}XCTest${CMAKE_SHARED_LIBRARY_SUFFIX}
         DESTINATION
           ${CMAKE_INSTALL_FULL_LIBDIR}/swift/${swift_os})
-
+if(NOT CMAKE_SYSTEM_NAME STREQUAL Darwin)
+  install(FILES
+            ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_SHARED_LIBRARY_PREFIX}XCTest${CMAKE_SHARED_LIBRARY_SUFFIX}
+          DESTINATION
+            ${CMAKE_INSTALL_FULL_LIBDIR}/swift/${swift_os}/${swift_host_arch})
+endif()