Use rust rather than LLVM target features in the target spec

[bjorn3]

This works better with non-LLVM codegen backends.

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

These commits modify compiler targets.
(See the Target Tier Policy.)

[rustbot]

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[RalfJung]

I haven't looked at the code yet, but I like the general direction!

The doc comment for https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/spec/struct.TargetOptions.html#structfield.features should also be updated.