[stdlib] Propagate SWIFT_STDLIB_SINGLE_THREADED_RUNTIME

This change may be unnecessary now

Author: kateinoigakukun

stdlib/cmake/modules/SwiftSource.cmake

@@ -327,6 +327,10 @@ function(_add_target_variant_swift_compile_flags
     list(APPEND result "-D" "SWIFT_STDLIB_HAS_STDIN")
   endif()
 
+  if(SWIFT_STDLIB_SINGLE_THREADED_RUNTIME)
+    list(APPEND result "-DSWIFT_STDLIB_SINGLE_THREADED_RUNTIME")
+  endif()
+
   if(SWIFT_STDLIB_HAS_ENVIRON)
     list(APPEND result "-D" "SWIFT_STDLIB_HAS_ENVIRON")
     list(APPEND result "-Xcc" "-DSWIFT_STDLIB_HAS_ENVIRON")