win64: add tiny unwind data for setjmp/longjmp

This enables native unwind semantics with longjmp on
win64 by putting an entry into the .pdata section for
each compiled fuction.

Also, the function now use a fixed stack and store arguments
into X(%rsp) rather than using push.

i386-asm.c

@@ -426,6 +426,10 @@ static void gen_disp32(ExprValue *pe)
            elimination in the linker */
         gen_le32(pe->v + sym->jnext - ind - 4);
     } else {
+        if (sym && sym->type.t == VT_VOID) {
+            sym->type.t = VT_FUNC;
+            sym->type.ref = NULL;
+        }
         gen_addrpc32(VT_SYM, sym, pe->v);
     }
 }