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acaa6cee07
rustc_target: Add RISC-V atomic-related features
This adds the following three target features to unstable riscv_target_feature.
- `zaamo` (Zaamo Extension 1.0.0): Atomic Memory Operations (`amo*.{w,d}{,.aq,.rl,.aqrl}`)
([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L229-L231), [available since LLVM 19](8be079cddd))
- `zabha` (Zabha Extension 1.0.0): Byte and Halfword Atomic Memory Operations (`amo*.{b,h}{,.aq,.rl,.aqrl}`)
([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L238-L240), [available since LLVM 19](6b7444964a))
- `zalrsc` (Zalrsc Extension 1.0.0): Load-Reserved/Store-Conditional Instructions (`lr.{w,d}{,.aq,.rl,.aqrl}` and `sc.{w,d}{,.aq,.rl,.aqrl}`)
([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L261-L263), [available since LLVM 19](8be079cddd))
(Zacas Extension is not included here because it is still marked as experimental in LLVM 19 70e7d26e56 and will become non-experimental in LLVM 20 614aeda93b)
`a` implies `zaamo` and `zalrsc`, and `zabha` implies `zaamo`:
- After Zaamo and Zalrsc Extensions are frozen, riscv-isa-manual says "The A extension comprises instructions provided by the Zaamo and Zalrsc extensions" (e87412e621), and [`a` implies `zaamo` and `zalrsc` in GCC](08693e29ec/gcc/config/riscv/arch-canonicalize (L44)). However, in LLVM, [`a` does not define them as implying `zaamo` and `zalrsc`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L206).
- Zabha and Zaamo are in a similar situation, [riscv-isa-manual](https://github.com/riscv/riscv-isa-manual/blob/main/src/zabha.adoc) says "The Zabha extension depends upon the Zaamo standard extension", and [`zabha` implies `zaamo` in GCC](08693e29ec/gcc/config/riscv/arch-canonicalize (L45-L46)), but [does not in LLVM (but enabling `zabha` without `zaamo` or `a` is not allowed)](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/TargetParser/RISCVISAInfo.cpp#L776-L778).
r? `@Amanieu`
`@rustbot` label +O-riscv +A-target-feature