win32: adjust new unicode support

- lib/Makefile: add (win)crt1_w.o

- crt1.c/_runtmain: return to tcc & only use for UNICODE
  (because it might be not 100% reliable with for example
  wildcards (tcc *.c -run ...)

- tccrun.c/tccpe.c: load -run startup_code only if called
  from tcc_run(). Otherwise main may not be defined.  See
  libtcc_test.c

- tests2/Makefile: pass extra options in FLAGS to allow
  overriding TCC

Also:
- tccpe.c: support weak attribute.  (I first tried to solve
  the problem above by using it but then didn't)

libtcc.c

@@ -743,9 +743,6 @@ LIBTCCAPI TCCState *tcc_new(void)
 #endif
 #ifdef TCC_TARGET_I386
     s->seg_size = 32;
-#endif
-#ifdef TCC_IS_NATIVE
-    s->runtime_main = "main";
 #endif
     /* enable this if you want symbols with leading underscore on windows: */
 #if 0 /* def TCC_TARGET_PE */