With GHC 8.4.1 we are now passing a default -mcpu selection to llvm. This default is passed regardless of any explicit command line option such as -optlc=-mcpu=native. If/when the user explicitly specifies the architecture we now get a failed build. The error message from LLVM is:

llc: for the -mcpu option: may only occur zero or one times!
`llc' failed in phase `LLVM Compiler'. (Exit code: 1)

This is when compiling with GHC 8.4.1 as such:

{{{ ghc-8.4.1 -fforce-recomp -fllvm -optlc=-mcpu=native -O2 tt.hs }}

I suggest we either check for a user option of mcpu before adding the default or revert this behavior and allow llvm to take it's (optimistic and less portable) default.