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`.

Author: jmillikin

library/std/src/os/net/mod.rs

@@ -1,4 +1,13 @@
 //! OS-specific networking functionality.
 
+// See cfg macros in `library/std/src/os/mod.rs` for why these platforms must
+// be special-cased during rustdoc generation.
+#[cfg(not(all(
+    doc,
+    any(
+        all(target_arch = "wasm32", not(target_os = "wasi")),
+        all(target_vendor = "fortanix", target_env = "sgx")
+    )
+)))]
 #[cfg(any(target_os = "linux", target_os = "android", doc))]
 pub(super) mod linux_ext;