Auto merge of #112910 - lqd:mcp510, r=petrochenkov

Implement most of MCP510

This implements most of what remains to be done for MCP510:
- turns `-C link-self-contained` into a `+`/`-` list of components, like `-C link-self-contained=+linker,+crto,+libc,+unwind,+sanitizers,+mingw`. The scaffolding is present for all these expected components to be implemented and stabilized in the future on their own time. This PR only handles the `-Zgcc-ld=lld` subset of these link-self-contained components as  `-Clink-self-contained=+linker`
- handles  `-C link-self-contained=y|n`  as-is today, for compatibility with `rustc_codegen_ssa::back::link::self_contained`'s [explicit opt-in and opt-out](https://github.com/lqd/rust/blob/9eee230cd0a56bfba3ce65121798d9f9f4341cdd/compiler/rustc_codegen_ssa/src/back/link.rs#L1671-L1676).
- therefore supports our plan to opt out of `rust-lld` (when it's enabled by default) even for current `-Clink-self-contained` users, with e.g. `-Clink-self-contained -Clink-self-contained=-linker`
- turns `add_gcc_ld_path` into its expected final form, by using the `-C link-self-contained=+linker`  CLI flag, and whether the `LinkerFlavor`  has the expected `Cc::Yes` and `Lld::Yes` shape (this is not yet the case in practice for any CLI linker flavor)
- makes the [new clean linker flavors](rust-lang/rust#96827 (comment)) selectable in the CLI in addition to the legacy ones, in order to opt-in to using `cc` and `lld` to emulate `-Zgcc-ld=lld`
- ensure the new `-C link-self-contained` components, and `-C linker-flavor`s are unstable, and require `-Z unstable-options` to be used

The up-to-date set of flags for the future stable CLI version of `-Zgcc-ld=lld` is currently: `-Clink-self-contained=+linker -Clinker-flavor=gnu-lld-cc -Zunstable-options`.

It's possible we'll also need to do something for distros that don't ship `rust-lld`, but maybe there are already no tool search paths to be added to `cc` in this situation anyways.

r? `@petrochenkov`

Author: bors