Add mechanism for adding/fetching target libc++ libs for x-compile

[phlax]

Currently the toolchain falls back to libstdc++ for any x-compile scenarios - iiuc on the basis that the host is more likely to have that available, and the libc++ in the toolchain won't work (wrong arch)

We (envoy) are currently hitting this for downstreams that want to x-compile to arm (cf envoyproxy/envoy#43826), and im about to add a workaround that bundles a package containing the arm libs/bins and then exposes them via a repo rule

Ideally this is something that could be handled inside toolchains_llvm directly - perhaps using some similar mechanism to how sysroot is pulled in currently

there is a related macos issue here #386

[keith]

fwiw this is solved by https://github.com/cerisier/toolchains_llvm_bootstrapped

[phlax]

hmm, interesting - im part way through implementing something, but probs we should consider it after bzlmod migration