MulMayOverflow test failures on x86-64

Looking at https://gitlab.haskell.org/ghc/test-primops/-/pipelines there are a lot of pipelines which have failed in the x86_64-linux-deb10-upstream stage (but because it's a quickcheck test sometimes it happens to pass). There are a lot of warnings from the assembler about constants that are being truncated so it feels like something's wrong with the code being generated somewhere.

Failing pipelines:

• https://gitlab.haskell.org/ghc/test-primops/-/pipelines/96043
• https://gitlab.haskell.org/ghc/test-primops/-/pipelines/95976 (also has a strange failure with the dyn package not being found for aarch64)
• https://gitlab.haskell.org/ghc/test-primops/-/pipelines/95974
• https://gitlab.haskell.org/ghc/test-primops/-/pipelines/95974

@AndreasK your last commit to this repo looks like it was trying to fix these issues?