[Build] Explicitly include threading library when not building stdlib.

The threading code is required by some of the compatibility modules,
but we weren't pulling it in unless we were building the standard library.
That breaks the build when the standard library build is disabled, so
explicitly include it.

rdar://90776105

Author: al45tair

CMakeLists.txt

@@ -1126,6 +1126,9 @@ endif()
 if(SWIFT_BUILD_STDLIB)
   add_subdirectory(stdlib)
 else()
+  # Some of the things below depend on the threading library
+  add_subdirectory(stdlib/public/Threading)
+
   if(SWIFT_BUILD_STDLIB_EXTRA_TOOLCHAIN_CONTENT)
     add_subdirectory(stdlib/toolchain)
   endif()

stdlib/public/Threading/CMakeLists.txt

@@ -1,6 +1,9 @@
 # This is the counterpart to lib/Threading/CMakeLists.txt.  Any updates
 # need to be reflected in both places.
 
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/../../cmake/modules")
+include(AddSwiftStdlib)
+
 add_swift_target_library(swiftThreading OBJECT_LIBRARY
   "${SWIFT_SOURCE_DIR}/lib/Threading/C11.cpp"
   "${SWIFT_SOURCE_DIR}/lib/Threading/Linux.cpp"