Cannot build ghc-8.8 with ghc-8.6

Summary

Trying to compile an unregisterised build of ghc-8.8 with ghc-8.6 fails due to missing symbol, stg_atomicModifyMutVarzh. This appears to have been replaced by stg_atomicModifyMutVar2zh (https://gitlab.haskell.org/ghc/ghc/-/blob/master/rts/PrimOps.cmm#L728).

Steps to reproduce

Build an unregisterised ghc-8.6, then build an unregisterised ghc-8.8 with ghc-8.6. Haven't checked if it's specific to an unregisterised build or not. Am using git tag ghc-8.8, and a cross-compiled ghc-8.6.5.

Expected behavior

Successfully build ghc-8.8

Environment

GHC version used: 8.6.5

Optional:

Operating System: Haiku
System Architecture: x86_64
Unregisterised haiku x86_64 ghc-8.6.5 toolchain: https://github.com/jessicah/haiku-ghc/releases/download/v8.6.5-1/ghc-8.6.5-x86_64-unknown-haiku.tar.xz

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