RUSTFLAGS="..." cargo .. should recompile std lib with flags enabled

[gnzlbg]

This is required for rust-san to work properly, and also for enabling some more efficient algorithms in std::simd that require specific target features (SSE4, AVX, NEON, etc.). Adding Iterator::is_sorted would also benefit from SSE4 and AVX. It currently does run-time feature detection but it could avoid that if std was compiled with the appropriate flags.

[Eh2406]

The current conversation is rust-lang/wg-cargo-std-aware#43
If that conversation is not the right issue, then that is the new place to open an issue.
Closing for now, so as to unify the conversations.