include/qemu/atomic.h: Add signal_barrier

We have some potential race conditions vs our user-exec signal
handler that will be solved with this barrier.

Backports commit 359896dfa4e9707e1acea99129d324250fccab04 from qemu

qemu/include/qemu/atomic.h

@@ -96,6 +96,13 @@ void _ReadWriteBarrier(void);
 #define smp_read_barrier_depends()   barrier()
 #endif
 
+/*
+ * A signal barrier forces all pending local memory ops to be observed before
+ * a SIGSEGV is delivered to the *same* thread.  In practice this is exactly
+ * the same as barrier(), but since we have the correct builtin, use it.
+ */
+#define signal_barrier()    __atomic_signal_fence(__ATOMIC_SEQ_CST)
+
 /* Sanity check that the size of an atomic operation isn't "overly large".
  * Despite the fact that e.g. i686 has 64-bit atomic operations, we do not
  * want to use them because we ought not need them, and this lets us do a
@@ -325,6 +332,10 @@ void _ReadWriteBarrier(void);
 #define smp_read_barrier_depends()   barrier()
 #endif
 
+#ifndef signal_barrier
+#define signal_barrier()    barrier()
+#endif
+
 /* These will only be atomic if the processor does the fetch or store
  * in a single issue memory operation
  */