They were introduced back when std_detect was a standalone crate
published to crates.io. The motivation for std_detect_dlsym_getauxval
was to allow using getauxval without dlopen when statically linking
musl, which we now unconditionally do for musl. And for
std_detect_file_io to allow no_std usage, which std_detect now supports
even with that feature enabled as it directly uses libc. This also
prevents accidentally disabling runtime feature detection when using
cargo build -Zbuild-std -Zbuild-std-features=
