LLVM default -mcpu setting inhibits customization

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.

Trac metadata

Trac field	Value
Version	8.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (LLVM)
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