experiment: use #[rustc_std_internal_symbol] to make #[no_mangle] tables have hidden visibility

[kkysen]

Based on @ahomescu's investigation:

• [Linux] calls to assembly routines have excessive indirection #1417 (comment)
• RFC: #[export_visibility = ...] attribute rust-lang/rfcs#3834
• rustc 1.88 regression: #[rustc_std_internal_symbol] #[no_mangle] is rejected rust-lang/rust#143422

I tried using #[rustc_std_internal_symbol] on rav1d's #[no_mangle] symbols (some of the tables in tables.rs). Per rust-lang/rust#143422, #[rustc_std_internal_symbol] is unstable and not meant to be used for this, and in fact, no longer works on #[no_mangle] symbols (so I had to move our pinned nightly back 1 day), but this is just for experimenting. This is able to successfully build as a cdylib with cargo build, not with cargo build --release, where I get error: undefined symbol for all of these symbols. So I don't think this works even for the hidden visibility issue (note: also fixed in #1367 through small changes to the assembly).

As for the performance issues raised in #1417, I don't think this addresses them. #1417 is about Rust calls to asm functions having unnecessary indirection, not about any symbols that Rust defines having any issues. And it can be fixed by using RUSTFLAGS="-Z plt=yes".

[kkysen]

Closing since this doesn't actually work and is very unstable so we wouldn't use it anyways.