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)

tccrun.c

@@ -107,6 +107,7 @@ LIBTCCAPI int tcc_run(TCCState *s1, int argc, char **argv)
 {
     int (*prog_main)(int, char **);
 
+    s1->runtime_main = "main";
     if (tcc_relocate(s1, TCC_RELOCATE_AUTO) < 0)
         return -1;
     prog_main = tcc_get_symbol_err(s1, s1->runtime_main);