sys/targets: add environment variable support for ARM64 triple config…

[happy2discover]

Instead of hard code ARM64 triple, a custom toolchain (e.g. aarch64-buildroot-linux-gnu) would be used sometimes

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[happy2discover]

@a-nogikh @tarasmadan could you please help to review? Thank you!

[tarasmadan]

Will symlink work for you?

[happy2discover]

Will symlink work for you?

@tarasmadan yes, created symlinks to aarch64-buildroot-linux-gnu can work.

[tarasmadan]

Then the proposal is to use symlinks instead.
My motivation: it will be much harder to find this flag that to symlink the binary.
If we want to continue with syzkaller, it is better to enable this env variable for all the targets and extend documentation to make this new flag searchable.
Do you agree?

[dvyukov]

If we want to continue with syzkaller, it is better to enable this env variable for all the targets and extend documentation to make this new flag searchable.

+1