Retire GCC FORTIFY.

NDK r18 (which removes GCC) is about to ship. We don't need this any more.

The immediate motivation for removing this right now is that the test
fails whenever we add Clang-only flags to the build system.

While we're here, clean up <stdatomic.h> too.

Bug: https://issuetracker.google.com/74404306
Test: ran tests
Change-Id: Iaad5d634d1ba76f0b6f19ad32cc27b2533771a4a