exec: elide calls to tb_lock and tb_unlock

Adding assertions fixes link errors.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -2317,6 +2317,7 @@ static void notdirty_mem_write(void *opaque, hwaddr ram_addr,
 {
     bool locked = false;
 
+    assert(tcg_enabled());
     if (!cpu_physical_memory_get_dirty_flag(ram_addr, DIRTY_MEMORY_CODE)) {
         locked = true;
         tb_lock();
@@ -2375,6 +2376,7 @@ static void check_watchpoint(int offset, int len, MemTxAttrs attrs, int flags)
     CPUWatchpoint *wp;
     uint32_t cpu_flags;
 
+    assert(tcg_enabled());
     if (cpu->watchpoint_hit) {
         /* We re-entered the check after replacing the TB. Now raise
          * the debug interrupt so that is will trigger after the
@@ -2820,6 +2822,7 @@ static void invalidate_and_set_dirty(MemoryRegion *mr, hwaddr addr,
             cpu_physical_memory_range_includes_clean(addr, length, dirty_log_mask);
     }
     if (dirty_log_mask & (1 << DIRTY_MEMORY_CODE)) {
+        assert(tcg_enabled());
         tb_lock();
         tb_invalidate_phys_range(addr, addr + length);
         tb_unlock();