ghc-prim: Link against libatomic
Commit b4d39adb made 'hs_cmpxchg64()' available to all architectures. Unfortunately this made GHC to fail to build on armel, since armel needs libatomic to support atomic operations on 64-bit word sizes.
Configure libraries/ghc-prim/ghc-prim.cabal to link against libatomic, the same way as we do in rts/rts.cabal.