Use "x86-64" as the target CPU for NetBSD and Bitrig on amd64.

[jakllsch]

Using "generic" disables a number of features that are present on all
x86_64 cpus, the "x86-64" target cpu is the common denominator for that
arch.

Refs #20777

[rust-highfive]

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[alexcrichton]

Thanks for the PR! I'm curious though, why not do this for all targets?

[jakllsch]

"All"? As far as I can tell it's already been done for every other x86_64-based system. The Apple ones use "core2" instead of "x86-64". On NetBSD there's a SSE-related test case failure this fixes because LLVM uses the generic lowest-common-denominator x86 instead of x86_64 which inherently has SSE2.

[nagisa]

I feel like it is a bug in LLVM that generic for x86_64 is not x86_64, but otherwise I’m not opposed to this PR.

[alexcrichton]

@bors: r+ 1f86005

Ah ok, thanks! I didn't realize we'd already done this for other targets, and sounds like good motivation to me.

[eddyb]

@bors r=alexcrichton rollup

[bors]

📌 Commit 990f193 has been approved by alexcrichton