Make test/Sema/atomic-ops.c free-standing

This test includes stdint.h, which might include system headers (and that might
not work, depending on the system configuration). Attempting to fix
llvm-clang-lld-x86_64-debian-fast.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218959 91177308-0d34-0410-b5e6-96231b3b80d8

test/Sema/atomic-ops.c

-// RUN: %clang_cc1 %s -verify -fsyntax-only -triple=i686-linux-gnu -std=c11
+// RUN: %clang_cc1 %s -verify -ffreestanding -fsyntax-only -triple=i686-linux-gnu -std=c11
 
 // Basic parsing/Sema tests for __c11_atomic_*