This is mostly copy-pasted from go/clang-fortify-anatomy. Since it
offers extensive documentation on how FORTIFY works in Bionic, having it
also live within Bionic seems quite helpful.
Bug: 235150687
Fixes: 235150687
Test: None
Change-Id: I20145a5ba3155b1c7b3977f9b688320a7fda4ea2
