rustc_target: RISC-V: add base I-related important extensions

[a4lg]

Of RISC-V features defined in stdarch, this commit adds ratified extensions satisfying following criteria:

• Formerly a part of the I extension and splitted thereafter (now ratified as I + Zifencei + Zicsr + Zicntr + Zihpm) or
• Dicoverable from newer versions of the Linux kernel (Zihintpause) and
• Available on LLVM 18.

This is based on the latest ratified ISA Manuals (version 20240411).

LLVM Definitions:

• Zifencei
• Zicsr
• Zicntr
• Zihpm
• Zihintpause

Additional (1):
One of those, Zicsr, is a dependency of many other ISA extensions and this commit adds correct dependencies to Zicsr except V (due to ongoing PR #138742).

Additional (2):
In RISC-V, G is an abbreviation of following extensions:

• I
• M
• A
• F
• D
• Zicsr (although implied by F)
• Zifencei

and all targets with either "riscv32gc" or "riscv64gc" are updated accordingly.

---

This is the version 3.
Ztso in the original proposal is removed on the PR version 2 due to the minimum LLVM version (non-experimental Ztso requires LLVM 19 while minimum LLVM version of Rust is 18). This is not back in PR version 3 after noticing adding Ztso is possible by checking host LLVM version because PR version 3 introduces compiler target changes (and adding more extensions would complicate the problems; sorry Zihintpause).

Related:

• Tracking issue for RFC 2045: improving #[target_feature] #44839
  (riscv_target_feature)
• Tracking Issue for RISC-V Ratified Extensions Intrinsics #114544
  (This PR can be a prerequisite of resolving a part of that tracking issue)
• rustc_target: Add more RISC-V vector-related features and use zvl*b target features in vector ABI check #138742
  (Touches the same place and vector extensions depend on Zicsr)

NOT Related but linked:

• Some -Ctarget-features must be restrained on RISCV #132618
  (This PR won't be blocked by this issue since none of those extensions do not change the ABI)

@rustbot r? @Amanieu
@rustbot label +T-compiler +O-riscv +A-target-feature

[rustbot]

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

[taiki-e]

and all targets with either "riscv32gc" or "riscv64gc" are updated accordingly.

I think the following two that do not include extension names in the target name are also actually riscv64gc.

rust/compiler/rustc_target/src/spec/targets/riscv64_linux_android.rs

Line 22 in 48994b1

features: "+m,+a,+f,+d,+c,+zba,+zbb,+zbs,+v".into(),

rust/compiler/rustc_target/src/spec/targets/riscv64_wrs_vxworks.rs

Line 19 in 48994b1

features: "+m,+a,+f,+d,+c".into(),

[Amanieu]

@a4lg Could you update the 2 targets mentioned by @taiki-e. Then this should be good to merge.