haskell.compiler.ghcHEAD: disable DWARF on non x86
haskell.compiler.ghcHEAD: disable DWARF on non x86

The broken build of ghcHEAD on aarch64-linux results from rts/Libdw.c
not supporting that platform. Seemingly this particular file is only
relevant for DWARF support in GHC, so we disable that on unsupported
platforms.

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