WIP armv8.1-a atomics preflight

Change-Id: Ie47203b9a3c0d69d500d9bb541727b75d0a1a18a
diff --git a/zircon/public/gn/config/BUILD.gn b/zircon/public/gn/config/BUILD.gn
index 3d177e8..bdaa08d 100644
--- a/zircon/public/gn/config/BUILD.gn
+++ b/zircon/public/gn/config/BUILD.gn
@@ -129,7 +129,7 @@
 
     if (current_cpu == "arm64") {
       # Generate code for the fairly generic cortex-a53.
-      compiler_flags += [ "-mcpu=cortex-a53" ]
+      compiler_flags += [ "-mcpu=cortex-a53", "-march=armv8.1-a" ]
 
       # x18 is reserved in the Fuchsia userland ABI so it can be used
       # for things like -fsanitize=shadow-call-stack.  In the kernel,