Undefined symbol atomic_inc64 when building GHC in devel1 flavour

When building a stage 1 compiler with Hadrian, I get a linker error involving atomic_inc64:

> hadrian/build.bat -j --flavour=devel1
ld.lld: error: undefined symbol: atomic_inc64
>>> referenced by libHSghc-9.9-inplace.a(genSym.o):(genSym)
clang: error: linker command failed with exit code 1 (use -v to see invocation)
`clang.exe' failed in phase `Linker'. (Exit code: 1)

This happens at the final linking step when building the GHC executable.

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