Rollup merge of #49225 - QuietMisdreavus:all-the-features-all-the-time, r=alexcrichton

Rollup merge of #49225 - QuietMisdreavus:all-the-features-all-the-time, r=alexcrichton

whitelist every target feature for rustdoc

When https://github.com/rust-lang-nursery/stdsimd/pull/367 was attempted to be upstreamed, it failed to document on non-x86 targets because it made every intrinsic visible, even the ones on foreign arches. This change makes it so that whenever rustdoc asks for the target feature whitelist, it gets a list of every feature known to every arch in `rustc_trans/llvm_util.rs`.

Before pushing, i temporarily updated the `stdsimd` submodule to include the `doc(cfg)` change, generated documentation for `aarch64-unknown-linux-gnu`, and it completed without a problem. The generated `core::arch` docs contained complete submodules for all main arches.