alpha-linux-user: Fix a3 error return with v0 error bypass.

We were failing to initialize a3 for syscalls that bypass the
negative return value error check.
Signed-off-by: Richard Henderson <rth@twiddle.net>

linux-user/main.c

@@ -2846,13 +2846,11 @@ void cpu_loop(CPUAlphaState *env)
                     break;
                 }
                 /* Syscall writes 0 to V0 to bypass error check, similar
-                   to how this is handled internal to Linux kernel.  */
-                if (env->ir[IR_V0] == 0) {
-                    env->ir[IR_V0] = sysret;
-                } else {
-                    env->ir[IR_V0] = (sysret < 0 ? -sysret : sysret);
-                    env->ir[IR_A3] = (sysret < 0);
-                }
+                   to how this is handled internal to Linux kernel.
+                   (Ab)use trapnr temporarily as boolean indicating error.  */
+                trapnr = (env->ir[IR_V0] != 0 && sysret < 0);
+                env->ir[IR_V0] = (trapnr ? -sysret : sysret);
+                env->ir[IR_A3] = trapnr;
                 break;
             case 0x86:
                 /* IMB */