llvmPackages_12.llvm: fix building on older CPUs

This commit patches one of the llvm-exegesis tests to swap out whatever
CPU model happens to be on the build host for bdver2 (AMD Family
15h/Piledriver), which was picked because it looks like that was the
intent of the test author. This provides a more predictable compilation
behaviour when running on older (or possibly even newer!) machines.

One of the machines that is currently part of the NixOS Hydra build
farm, wendy, is using an old AMD Opteron CPU for which LLVM has no
scheduling machine model. This causes one of the tests for llvm-exegesis
to fail, because it segfaults trying to use the machine model to produce
useful analysis results.

Note that this particular test only runs on x86-64 build hosts anyway;
aarch64 isn't affected.

This deliberately only patches LLVM 12 to limit the rebuilds; other
LLVM versions are going through staging.

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