Document stack-protector option

Only updated `exploit-mitigations.md` to reflect that the option exists. Removed the alternatives
mentioned as they are not actually implemented yet.

As this is an unstable feature, should it be added to `unstable-book` also? I didn't do that because
I couldn't find the tracking issue for it. (There should be one to track stabilization of the
feature.)

Author: mrcnski

src/doc/rustc/src/exploit-mitigations.md

@@ -115,9 +115,9 @@ equivalent.
   <tr>
    <td>Stack smashing protection
    </td>
-   <td>No
+   <td>Yes
    </td>
-   <td>
+   <td>Nightly
    </td>
   </tr>
   <tr>
@@ -432,9 +432,7 @@ saved return instruction pointer, and checking if this value has changed
 when returning from a function. This is also known as “Stack Protector” or
 “Stack Smashing Protector (SSP)”.
 
-The Rust compiler does not support stack smashing protection. However, more
-comprehensive alternatives to stack smashing protection exist, such as
-shadow and safe stack (see backward-edge control flow protection).
+The Rust compiler supports stack smashing protection on nightly builds[42].
 
 ![Screenshot of IDA Pro listing cross references to __stack_chk_fail in hello-rust.](images/image3.png "Cross references to __stack_chk_fail in hello-rust.")
 Fig. 14. IDA Pro listing cross references to `__stack_chk_fail` in
@@ -697,3 +695,6 @@ defaults (unrelated to `READ_IMPLIES_EXEC`).
 
 41. “ControlFlowIntegrity.” The Rust Unstable Book.
     [https://doc.rust-lang.org/unstable-book/compiler-flags/sanitizer.html#controlflowintegrity](../unstable-book/compiler-flags/sanitizer.html#controlflowintegrity).
+
+42. bbjornse. “add codegen option for using LLVM stack smash protection #84197.”
+    GitHub. <https://github.com/rust-lang/rust/pull/84197>