Revert "Reland "[llvm-exegesis] Switch from MCJIT to LLJIT (#72838)""

This broke

  tools/llvm-exegesis/X86/latency/dummy-counters.test

on Mac, see comment on the PR.

> There was an issue with certain configurations failing to build due to a
> deleted Error constructor that should be fixed with this relanding.

This reverts commit 92b821f2.