tcg: Add EXCP_ATOMIC

When we cannot emulate an atomic operation within a parallel
context, this exception allows us to stop the world and try
again in a serial context.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
diff --git a/cpu-exec-common.c b/cpu-exec-common.c
index 0cb4ae6..767d9c6 100644
--- a/cpu-exec-common.c
+++ b/cpu-exec-common.c
@@ -77,3 +77,9 @@
     }
     siglongjmp(cpu->jmp_env, 1);
 }
+
+void cpu_loop_exit_atomic(CPUState *cpu, uintptr_t pc)
+{
+    cpu->exception_index = EXCP_ATOMIC;
+    cpu_loop_exit_restore(cpu, pc);
+}