build: additional section_magic conditional fix

The variable for the TARGET_LIBRARY option should be
SWIFTLIB_SINGLE_TARGET_LIBRARY.

Author: adierking

cmake/modules/AddSwift.cmake

@@ -719,7 +719,7 @@ function(_add_swift_library_single target name)
   # On platforms that use ELF binaries we add markers for metadata sections in
   # the shared libraries using these object files.  This wouldn't be necessary
   # if the link was done by the swift binary: rdar://problem/19007002
-  if(SWIFTLIB_TARGET_LIBRARY AND
+  if(SWIFTLIB_SINGLE_TARGET_LIBRARY AND
      "${SWIFT_SDK_${SWIFTLIB_SINGLE_SDK}_OBJECT_FORMAT}" STREQUAL "ELF")
     if("${libkind}" STREQUAL "SHARED")
       set(arch_subdir "${SWIFTLIB_DIR}/${SWIFTLIB_SINGLE_SUBDIR}")