Add MADV_SOFT_OFFLINE definition for RISC-V musl targets

[mnissler-rivos]

Description

This adds missing MADV_SOFT_OFFLINE definitions for RISC-V musl targets. It is analogous to #2391 where the same definitions were added for RISC-V glibc targets. The patch addresses a rustix build issue: bytecodealliance/rustix#1462

Sources

https://github.com/bminor/musl/blob/c47ad25ea3b484e10326f933e927c0bc8cded3da/include/sys/mman.h#L99

Checklist

• Relevant tests in libc-test/semver have been updated
  -- I have added MADV_SOFT_OFFLINE to libc-test/semver/linux-riscv64gc.txt for good measure
• No placeholder or unstable values like *LAST or *MAX are
  included (see rust-lang/libc#3131)
• Tested locally (cd libc-test && cargo test --target riscv64gc-unknown-linux-{gnu,musl});
  especially relevant for platforms that may not be checked in CI
  -- I've tested the riscv64gc-unknown-linux-musl target against a riscv-gnu-toolchain musl build - there are actually a bunch of other issues with struct layout, field name, and value inconsistencies, but these are unrelated to my change.

@rustbot label +stable-nominated

[mnissler-rivos]

There's one consideration I want to bring up: Instead of adding the missing definitions to the RISC-V musl targets, we could also opt for a single definition at linux_like level, with cfg guards to exempt mips platforms. The definition was actually more centralized, but 1cbc523 changed it to the approach that is currently used, and I don't know what the rationale/preference is. Let me know if you'd prefer a conditionally compiled definition in linux_like and I'll happily update the pull request.

[tgross35]

There's one consideration I want to bring up: Instead of adding the missing definitions to the RISC-V musl targets, we could also opt for a single definition at linux_like level, with cfg guards to exempt mips platforms. The definition was actually more centralized, but 1cbc523 changed it to the approach that is currently used, and I don't know what the rationale/preference is. Let me know if you'd prefer a conditionally compiled definition in linux_like and I'll happily update the pull request.

I don't like this repo's layout :) we'll be reorganizing things soon, don't worry about it for now.