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
diff --git a/libc/include/string.h b/libc/include/string.h
index 54d5e1c..25f6673 100644
--- a/libc/include/string.h
+++ b/libc/include/string.h
@@ -144,7 +144,7 @@
 #endif
 
 /* Const-correct overloads. Placed after FORTIFY so we call those functions, if possible. */
-#if defined(__cplusplus) && defined(__clang__)
+#if defined(__cplusplus)
 /*
  * Use two enable_ifs so these overloads don't conflict with + are preferred over libcxx's. This can
  * be reduced to 1 after libcxx recognizes that we have const-correct overloads.