add u-boot crosstool to fix u-boot can not build by buildroot crosstool 7.0...

guowenxue

2019-08-24 a6177b46d08f140c3287e79d4b99f78621ddfa5e

 tools/gcc-linaro-4.9.4-2017.01-x86_64_arm-linux-gnueabi.tar.xz

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