Rollup merge of #86327 - GuillaumeGomez:safe-intrinsics, r=lqd

Don't mark "safe" intrinsics as unsafe

A good example of this is [intrinsics::abort](https://doc.rust-lang.org/nightly/core/intrinsics/fn.abort.html).

Before:

![Screenshot from 2021-06-15 14-58-42](https://user-images.githubusercontent.com/3050060/122056942-65ddad00-cdea-11eb-829e-5f5e258387de.png)

After:

![Screenshot from 2021-06-15 14-59-22](https://user-images.githubusercontent.com/3050060/122056956-6aa26100-cdea-11eb-94d8-e18b4956cfa4.png)

cc ``@jyn514``
r? ``@lqd``

src/test/rustdoc/safe-intrinsic.rs

@@ -0,0 +1,20 @@
+#![feature(intrinsics)]
+#![feature(no_core)]
+
+#![no_core]
+#![crate_name = "foo"]
+
+extern "rust-intrinsic" {
+    // @has 'foo/fn.abort.html'
+    // @has - '//pre[@class="rust fn"]' 'pub extern "rust-intrinsic" fn abort() -> !'
+    pub fn abort() -> !;
+    // @has 'foo/fn.unreachable.html'
+    // @has - '//pre[@class="rust fn"]' 'pub unsafe extern "rust-intrinsic" fn unreachable() -> !'
+    pub fn unreachable() -> !;
+}
+
+extern "C" {
+    // @has 'foo/fn.needs_drop.html'
+    // @has - '//pre[@class="rust fn"]' 'pub unsafe extern "C" fn needs_drop() -> !'
+    pub fn needs_drop() -> !;
+}