common/rc: generalize _get_filesize()

common/rc: generalize _get_filesize()

There are some testcases use below two kind of commands to get file
size, generalize the second way as global function _get_filesize()
to simply codes.

1. ls -l $1 | $AWK_PROG '{print $5}'
2. stat -c %s $1

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>