llvm-core: add cross-compilation support

[valgur]

Summary

Changes to recipe: llvm-core/[*]

Motivation

Enable cross-compilation of the recipe.

Details

Uses GnuToolchain to set the required triplet value.

Tested with gcc-13-aarch64-linux-gnu: https://gist.github.com/valgur/5a550530a55b0df98016b89dbb25d861

• static: llvm-core.log
• shared: llvm-core.log

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• Checked that this PR is not a duplicate: list of PRs by recipe
• Tested locally with at least one configuration using a recent version of Conan

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