Coordinate with "std-aware Cargo" WG

[cher-nov]

Although this initiative currently looks somewhat stalled, I decided to mention this collaboration opportunity anyway, just like #9 and #10 do. See this for rationale: https://rust-lang.github.io/rust-project-goals/2025h1/build-std.html

In theory, some portability issues could be solved by simply rebuilding the standard library, disabling the unsupported functionality or switching to alternative code paths. The set of *-win7-windows-msvc tier-3 targets is pretty good example of this. Another one is Rust9x project. This is exactly what build-std is for, among other things.

See also:

• https://github.com/rust-lang/wg-cargo-std-aware/issues?q=state%3Aopen label%3A"use case"
• Add min_$TARGET_API_version cfg predicates rust-lang/rfcs#3036
• https://rust-lang.github.io/rfcs/2396-target-feature-1.1.html