tcctest: add back testXb (self compile with -b)

- Thanks to Kirill "tcc -b itself" should work now
  (was removed in d5f4df09ff)

Also:

- tests/Makefile:
  - fix spurious --I from 767410b875
  - lookup boundtest.c via VPATH (for out-of-tree build)
  - test[123]b?: fail on diff error
  - Windows: test3 now works (from e31579b076)
  - abitest: a libtcc.a made by gcc is not usable for tcc
    on WIndows - using source instead (libtcc.c)

- tccpe:
  - avoid gcc warning (x86_64)

Makefile

@@ -346,8 +346,8 @@ tcc-doc.info: tcc-doc.texi
 # in tests subdir
 export LIBTCC1
 
-%est:
-	$(MAKE) -C tests $@ "PROGS_CROSS=$(PROGS_CROSS)"
+test test% %test :
+	$(MAKE) -C tests $@ 'PROGS_CROSS=$(PROGS_CROSS)'
 
 clean:
 	rm -vf $(PROGS) tcc_p$(EXESUF) tcc.pod *~ *.o *.a *.so* *.out *.exe libtcc_test$(EXESUF)