Proposal: remove `linkme`

[AsakuraMizu]

linkme is a tool for collecting elements across crates during linking. We use it to maintain trap callbacks.

Why

However, there are currently the following issues:

1. The trap callback slice we create using linkme will only have 0 or 1 items. It's somewhat overkill.

2. It doesn't work well on bare metal (target_os = "none"), because bare metal applications typically require writing a linker script and then maintaining each used linkme section (linkme_XXX/linkm2_XXX), otherwise it will become Orphan Sections, and then be placed in a (potentially inappropriate) location by the linker.

Possible Alternatives

1. Use crate_interface or similar methods to leverage weak symbols to define trap callbacks.
2. Patch linkme and rename the generated symbols to .rodata.xxx, etc., which might solve the problem (unverified).

[AsakuraMizu]

Another attractive implementation is extern_item_impls. Although it was only added to the nightly releases a few months ago, it does have a significant advantage: it has compiler-level support and a smoother, cleaner user experience.

Popular projects like getrandom (the underlying implementation of rand) have already used it as an experimental feature.

[AsakuraMizu]

Update: During our experiments on EII, we discovered a bug in the current implementation of rustc. A PR (rust-lang/rust#153648) has been submitted for a fix and is currently awaiting merging. After applying the patch, the proposal was successfully implemented.