I fixed the problem by removing Adobe AIR.framework from Build Phases > Link Binary With Libraries section in XCode. You only need to include Adobe AIR_64. Under Build Settings > Architectures choose Standard Architecture (64bit-Intel) (x86_64). The resulting library only supports 64bit platforms but that shouldn't be an issue according to this post. I am using FlashBuilder 4.7 with Flex 4.15.0 + AIR 21.0 SDK.
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