Rollup merge of #106618 - jmillikin:os-net-rustdoc-wasm32, r=JohnTitor

Disable `linux_ext` in wasm32 and fortanix rustdoc builds.

The `std::os::unix` module is stubbed out when building docs for these target platforms. The introduction of Linux-specific extension traits caused `std::os::net` to depend on sub-modules of `std::os::unix`, which broke rustdoc for the `wasm32-unknown-unknown` target.

Adding an additional `#[cfg]` guard solves that rustdoc failure by not declaring `linux_ext` on targets with a stubbed `std::os::unix`.

Fixes #105467