tccgen: arm/i386: save_reg_upstack

tccgen.c:gv() when loading long long from lvalue, before
was saving all registers which caused problems in the arm
function call register parameter preparation, as with

    void foo(long long y, int x);
    int main(void)
    {
      unsigned int *xx[1], x;
      unsigned long long *yy[1], y;
      foo(**yy, **xx);
      return 0;
    }

Now only the modified register is saved if necessary,
as in this case where it is used to store the result
of the post-inc:

        long long *p, v, **pp;
        v = 1;
        p = &v;
        p[0]++;
        printf("another long long spill test : %lld\n", *p);

i386-gen.c :
- found a similar problem with TOK_UMULL caused by the
  vstack juggle in tccgen:gen_opl()
  (bug seen only when using EBX as 4th register)

i386-gen.c

@@ -848,6 +848,8 @@ ST_FUNC void gen_opi(int op)
         fr = vtop[0].r;
         vtop--;
         save_reg(TREG_EDX);
+        /* save EAX too if used otherwise */
+        save_reg_upstack(TREG_EAX, 1);
         if (op == TOK_UMULL) {
             o(0xf7); /* mul fr */
             o(0xe0 + fr);