Rollup merge of #83820 - petrochenkov:nolinkargs, r=nagisa

Remove attribute `#[link_args]`

Closes https://github.com/rust-lang/rust/issues/29596

The attribute could always be replaced with `-C link-arg`, but cargo didn't provide a reasonable way to pass such flags to rustc.
Now cargo supports `cargo:rustc-link-arg*` directives in build scripts (https://doc.rust-lang.org/cargo/reference/unstable.html#extra-link-arg), so this attribute can be removed.

src/doc/unstable-book/src/language-features/link-args.md

@@ -1,32 +0,0 @@
-# `link_args`
-
-The tracking issue for this feature is: [#29596]
-
-[#29596]: https://github.com/rust-lang/rust/issues/29596
-
-------------------------
-
-You can tell `rustc` how to customize linking, and that is via the `link_args`
-attribute. This attribute is applied to `extern` blocks and specifies raw flags
-which need to get passed to the linker when producing an artifact. An example
-usage would be:
-
-```rust,no_run
-#![feature(link_args)]
-
-#[link_args = "-foo -bar -baz"]
-extern "C" {}
-# fn main() {}
-```
-
-Note that this feature is currently hidden behind the `feature(link_args)` gate
-because this is not a sanctioned way of performing linking. Right now `rustc`
-shells out to the system linker (`gcc` on most systems, `link.exe` on MSVC), so
-it makes sense to provide extra command line arguments, but this will not
-always be the case. In the future `rustc` may use LLVM directly to link native
-libraries, in which case `link_args` will have no meaning. You can achieve the
-same effect as the `link_args` attribute with the `-C link-args` argument to
-`rustc`.
-
-It is highly recommended to *not* use this attribute, and rather use the more
-formal `#[link(...)]` attribute on `extern` blocks instead.