Tracking Issue for fine-grained `-C link-self-contained`

[petrochenkov]

This is a tracking issue for the unstable parts of the command line option -C link-self-contained=+component1,-component2.
There's no RFC, but some of the feature was stabilized by an FCP for LLD-by-default.

The stable subset is documented in https://doc.rust-lang.org/rustc/codegen-options/index.html#link-self-contained.
The unstable and mostly unimplemented subset is documented in https://doc.rust-lang.org/beta/unstable-book/compiler-flags/codegen-options.html#link-self-contained.

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Steps

• Support other components besides linker in rustc_codegen_ssa and maybe other places
• Move all the libraries and binaries to subdirectories of the self-contained directories corresponding to the individual components
• Move self-contained components not currently living in the self-contained directories to those directories
  • Move libunwind
  • Move mingw linker and C compiler
  • Move sanitizers
• Migrate targets using some shipped components without considering the self-contained mechanism to that mechanism
  • libunwind on some targets
  • linker and C compiler on mingw
  • sanitizers everywhere
• Migrate bare-metal targets using rust-lld as linker to using ld.lld through the self-contained mechanism
• On targets where -C link-self-contained=+something/yes does nothing it should be prohibited or unstable
• On targets where -C link-self-contained=-something/no does nothing and the target behaves correctly with regards to self-contained (rust-lld/libunwind/etc are migrated), it can be allowed and stabilized.
• On targets where -C link-self-contained doesn't behave correctly (rust-lld/libunwind/etc are not migrated), it should be prohibited or unstable
• On targets where -C link-self-contained=something does something it can be allowed and stabilized through an FCP or MCP mechanism
• Perhaps rename self-contained directories to sc to shorten all the paths

Unresolved Questions

• TBD

Implementation history

• TODO

Some relevant PRs

• Implement unstable -Clink-self-contained values for MCP 510 #96884 (initial design for the component list)
• Implement most of MCP510 #112910 (initial implementation)
• Implement -Clink-self-contained=-linker opt out #116014
• Allow target specs to use an LLD flavor, and self-contained linking components #116035
• Document unstable linker flags in the unstable book #119024
• Use lld by default on x86_64-unknown-linux-gnu stable #140525