Rollup merge of #125263 - lqd:lld-fallback, r=petrochenkov

rust-lld: fallback to rustc's sysroot if there's no path to the linker in the target sysroot

As seen in #125246, some sysroots don't expect to contain `rust-lld` and want to keep it that way, so we fallback to the default rustc sysroot if there is no path to the linker in any of the sysroot tools search paths. This is how we locate codegen-backends' dylibs already.

People also have requested an error if none of these search paths contain the self-contained linker directory, so there's also an error in that case.

r? `@petrochenkov` cc `@ehuss` `@RalfJung`

I'm not sure where we check for `rust-lld`'s existence on the targets where we use it by default, and if we just ignore it when missing or emit a warning (as I assume we don't emit an error), so I just checked for the existence of `gcc-ld`, where `cc` will look for the lld-wrapper binaries.

<sub>*Feel free to point out better ways to do this, it's the middle of the night here.*</sub>

Fixes #125246

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