Allows to provide Lipo tool location via custom toolchain.

[DmitriiTcimbal]

There might be cases when universal_binary rule has to be used in the absence of a local Xcode/CLT installation. This MR allows the location of the Lipo tool to be provided via the custom toolchain.

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[keith]

I don't think it's possible to use this rule without xcode installed since it relies on this?

apple_support/lib/apple_support.bzl

Line 223 in 19cbebf

"_xcode_config": attr.label(

[keith]

I think im wrong about that and it must be a no-op for linux at the very least. at first i wasn't thinking we should merge this but i've since seen that grpc started using this rule, and if you want to avoid major xcode version dependence you need this, so i think we should probably move forward with it. can you rebase and sign the CLA?