tcg: fix --disable-tcg build breakage

Fix the --disable-tcg breakage introduced by tb_lock's removal by
relying on the fact that tcg_enabled() is set to 0 at
compile-time under --disable-tcg.

While at it, add further asserts to fix builds that enable both
--disable-tcg and --enable-debug, which were broken even before
tb_lock's removal.

Tested to build x86_64-softmmu and i386-softmmu targets.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

exec.c

@@ -1323,6 +1323,7 @@ static void tlb_reset_dirty_range_all(ram_addr_t start, ram_addr_t length)
     RAMBlock *block;
     ram_addr_t end;
 
+    assert(tcg_enabled());
     end = TARGET_PAGE_ALIGN(start + length);
     start &= TARGET_PAGE_MASK;
 
@@ -2655,6 +2656,7 @@ void memory_notdirty_write_prepare(NotDirtyInfo *ndi,
 void memory_notdirty_write_complete(NotDirtyInfo *ndi)
 {
     if (ndi->pages) {
+        assert(tcg_enabled());
         page_collection_unlock(ndi->pages);
         ndi->pages = NULL;
     }
@@ -3046,6 +3048,7 @@ static void tcg_commit(MemoryListener *listener)
     CPUAddressSpace *cpuas;
     AddressSpaceDispatch *d;
 
+    assert(tcg_enabled());
     /* since each CPU stores ram addresses in its TLB cache, we must
        reset the modified entries */
     cpuas = container_of(listener, CPUAddressSpace, tcg_as_listener);