Make swift_async_extendedFramePointerFlags = 0 in back-deployed configurations.
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.

roambotics/swift,rudkx/swift,ahoppen/swift,benlangmuir/swift,atrick/swift,gregomni/swift,gregomni/swift,ahoppen/swift,roambotics/swift,gregomni/swift,rudkx/swift,JGiola/swift,roambotics/swift,glessard/swift,JGiola/swift,atrick/swift,apple/swift,rudkx/swift,JGiola/swift,ahoppen/swift,xwu/swift,atrick/swift,glessard/swift,apple/swift,apple/swift,atrick/swift,glessard/swift,apple/swift,benlangmuir/swift,gregomni/swift,xwu/swift,roambotics/swift,xwu/swift,xwu/swift,ahoppen/swift,atrick/swift,glessard/swift,JGiola/swift,ahoppen/swift,glessard/swift,benlangmuir/swift,atrick/swift,apple/swift,JGiola/swift,gregomni/swift,benlangmuir/swift,ahoppen/swift,xwu/swift,rudkx/swift,roambotics/swift,rudkx/swift,xwu/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,roambotics/swift,apple/swift,rudkx/swift,gregomni/swift,xwu/swift,JGiola/swift