[U-Boot] how to get u-boot code with arm64: core support

[TigerLiu at viatech.com.cn]

Hi, Inder:

>aarch64-linux-gnu-gcc: error: unrecognized command line option '-marm'
>aarch64-linux-gnu-gcc: error: unrecognized command line option
'-mno-thumb-interwork'
>aarch64-linux-gnu-gcc: error: unrecognized command line option
'-mabi=aapcs-linux'
>aarch64-linux-gnu-gcc: error: unrecognized command line option
'-mword-relocations'

I didn't run into this issue.