target-alpha: Add various symbolic constants.

The EXC_M_* constants were being set for the EV6, not as set for
the Unix kernel entry point.

Use PS_USER_MODE instead of hard-coding access to the PS register.

Signed-off-by: Richard Henderson <rth@twiddle.net>
diff --git a/target-alpha/translate.c b/target-alpha/translate.c
index 5f40d34..2f3c637 100644
--- a/target-alpha/translate.c
+++ b/target-alpha/translate.c
@@ -3269,7 +3269,7 @@
     env->amask = amask;
 
 #if defined (CONFIG_USER_ONLY)
-    env->ps = 1 << 3;
+    env->ps = PS_USER_MODE;
     cpu_alpha_store_fpcr(env, (FPCR_INVD | FPCR_DZED | FPCR_OVFD
                                | FPCR_UNFD | FPCR_INED | FPCR_DNOD));
 #endif