tcg: Prepare TB invalidation for lockless TB lookup

When invalidating a translation block, set an invalid flag into the
TranslationBlock structure first.  It is also necessary to check whether
the target TB is still valid after acquiring 'tb_lock' but before calling
tb_add_jump() since TB lookup is to be performed out of 'tb_lock' in
future. Note that we don't have to check 'last_tb'; an already invalidated
TB will not be executed anyway and it is thus safe to patch it.

Suggested-by: Sergey Fedorov <serge.fdrv@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/exec/exec-all.h

@@ -225,6 +225,8 @@ struct TranslationBlock {
 #define CF_USE_ICOUNT  0x20000
 #define CF_IGNORE_ICOUNT 0x40000 /* Do not generate icount code */
 
+    uint16_t invalid;
+
     void *tc_ptr;    /* pointer to the translated code */
     uint8_t *tc_search;  /* pointer to search data */
     /* original tb when cflags has CF_NOCACHE */