Add exploit mitigations chapter to the rustc book

This section documents the exploit mitigations applicable to the Rust
compiler when building programs for the Linux operating system on the
AMD64 architecture and equivalent.

Author: rcvalle

src/doc/rustc/src/SUMMARY.md

@@ -18,4 +18,5 @@
     - [Known Issues](targets/known-issues.md)
 - [Profile-guided Optimization](profile-guided-optimization.md)
 - [Linker-plugin based LTO](linker-plugin-lto.md)
+- [Exploit Mitigations](exploit-mitigations.md)
 - [Contributing to `rustc`](contributing.md)