scripts: use readelf instead of file to check elf is riscv

file in centos 6.10 is 5.04, which is not able to tell this executable
is a riscv version, so we change to use readelf now

the version readelf in centos 6.10 is 2.30 which works to tell
executable is a riscv one

Signed-off-by: Huaqi Fang <578567190@qq.com>