GHC 8.10 needs libatomic on Debian armel

After fixing #20285, GHC 8.10 fails to build on Debian armel with the following error:

rts/dist/build/libHSrts_thr-ghc8.10.7.so: error: undefined reference to '__atomic_load_8'
rts/dist/build/libHSrts_thr-ghc8.10.7.so: error: undefined reference to '__atomic_store_8'
rts/dist/build/libHSrts_thr-ghc8.10.7.so: error: undefined reference to '__atomic_fetch_add_8'

Looking at the code, I can see at least one place that RELAXED_LOAD (i.e., __atomic_load_n) is used on an 64-bit variable (see https://gitlab.haskell.org/ghc/ghc/-/blob/wip/8.10.7-release/rts/STM.c#L905), which is not supported on armel without using libatomic.

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