cpufeatures: `cfg_select!`-like macro

More than once I wrote a piece of code that has the following shape:

cpufeatures::new!(has_avx512f_vaes, "avx512f", "vaes");
if has_avx512f_vaes::get() {
    return /* .. */;
}

cpufeatures::new!(has_avx2_vaes, "avx2", "vaes");
if has_avx2_vaes::get() {
    return /* .. */;
}

cpufeatures::new!(has_aes_sse41, "aes", "sse4.1");
if has_aes_sse41::get() {
    return /* .. */;
}

// Fallback

It'd be great to have something that can replace multiple individual atomic reads with a single one in such cases.

The shape of cfg_select! seems perfect for this.

Start at the cpufeatures::new! entry point and compare the requested behavior with Rust's cfg_select! macro semantics. Trace how generated feature checks are read, then define and test an ordered selection interface that avoids repeated atomic reads and preserves the fallback; done when the requested feature combinations select the correct branch.