Include host compatibility library build directory

The SDK doesn't have the new compatibility library and the
crosscompile-with-hostlibs bootstrap configuration is failing.

This bootstrap configuration is the only one that does not fall back on
the host built compatibility libraries if it is not available in the
SDK.

Author: etcwilde

cmake/modules/AddSwift.cmake

@@ -471,6 +471,10 @@ function(_add_swift_runtime_link_flags target relpath_to_lib_dir bootstrapping)
       # Add the SDK directory for the host platform.
       target_link_directories(${target} PRIVATE "${sdk_dir}")
 
+      # A backup in case the toolchain doesn't have one of the compatibility libraries.
+      target_link_directories(${target} PRIVATE
+        "${SWIFTLIB_DIR}/${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_LIB_SUBDIR}")
+
       # Include the abi stable system stdlib in our rpath.
       set(swift_runtime_rpath "/usr/lib/swift")