Make swift_async_extendedFramePointerFlags = 0 in back-deployed configurations.

`swift_async_extendedFramePointerFlags` is used in back-deployed
concurrency code as a mask to set the extended async frame info bit on
OS versions that understand it. Set this mask to zero in the
back-deployed libraries.

Author: DougGregor

stdlib/public/Concurrency/Task.cpp

@@ -33,6 +33,10 @@
 #include <dlfcn.h>
 #endif
 
+#if defined(SWIFT_CONCURRENCY_BACK_DEPLOYMENT)
+asm("\n .globl _swift_async_extendedFramePointerFlags" \
+    "\n _swift_async_extendedFramePointerFlags = 0x0");
+#else
 #ifdef __APPLE__
 #if __POINTER_WIDTH__ == 64
 asm("\n .globl _swift_async_extendedFramePointerFlags" \
@@ -45,6 +49,7 @@ asm("\n .globl _swift_async_extendedFramePointerFlags" \
     "\n _swift_async_extendedFramePointerFlags = 0x0");
 #endif
 #endif // __APPLE__
+#endif // !defined(SWIFT_CONCURRENCY_BACK_DEPLOYMENT)
 
 using namespace swift;
 using FutureFragment = AsyncTask::FutureFragment;