rustc: Whitelist the FMA target feature

[fsasm]

This commit adds the entry "fma\0" to the whitelist for the x86
target. LLVM already supports fma but rustc did not directly. Previously
rustc permitted +fma in the target-feature argument and enabled the use
of FMA instructions, but it did not list it in the configuration and
attributes.

fixes #40406

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @eddyb (or someone else) soon.

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[eddyb]

cc @rust-lang/compiler @rust-lang/libs What's our policy here?

[BurntSushi]

@eddyb We don't have a policy. We've just been kind of merging them as they come up.

@fsasm What are you using this for?

[fsasm]

@BurntSushi I'm trying to add FMA support to the simd crate. Because FMA can improve precision I want to use it later for some scientific computations.

[BurntSushi]

@bors r+

[bors]

📌 Commit 4d23ca4 has been approved by BurntSushi