linux-user: enable parallel code generation on clone

The variable parallel_cpus controls the generation of thread aware
atomic code.  We only need to set it once we clone our first thread.
At this point any existing translations need to be thrown away.
Reviewed-by: NEmilio G. Cota <cota@braap.org>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NRichard Henderson <rth@twiddle.net>

linux-user/syscall.c

@@ -6164,6 +6164,14 @@ static int do_fork(CPUArchState *env, unsigned int flags, abi_ulong newsp,
         sigfillset(&sigmask);
         sigprocmask(SIG_BLOCK, &sigmask, &info.sigmask);
 
+        /* If this is our first additional thread, we need to ensure we
+         * generate code for parallel execution and flush old translations.
+         */
+        if (!parallel_cpus) {
+            parallel_cpus = true;
+            tb_flush(cpu);
+        }
+
         ret = pthread_create(&info.thread, &attr, clone_func, &info);
         /* TODO: Free new CPU state if thread creation failed.  */