[libc] Fix the fputc issue when use microlib.

components/libc/compilers/armlibc/stubs.c

@@ -313,17 +313,18 @@ int system(const char *string)
 
 int fputc(int c, FILE *f) 
 {
-    char ch = c;
+    char ch[2] = {0};
 
-    rt_kprintf(&ch);
+    ch[1] = c;
+    rt_kprintf(&ch[0]);
     return 1;
 }
 
 int fgetc(FILE *f) 
 {
+#ifdef RT_USING_POSIX
     char ch;
 
-#ifdef RT_USING_POSIX
     if (libc_stdio_read(&ch, 1) == 1)
         return ch;
 #endif