stabilize `-Clink-self-contained=-linker` on x64 linux

This stabilizes a subset of the `-Clink-self-contained` components on x64 linux:
the rust-lld opt-out.

The opt-in is not stabilized, as interactions with other stable flags require
more internal work, but are not needed for stabilizing using rust-lld by default.

Similarly, since we only switch to rust-lld on x64 linux, the opt-out is
only stabilized there. Other targets still require `-Zunstable-options`
to use it.

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