Adds a GN template for instantiating groups whose dependencies will be
built by multiple toolchains. This is useful for simplifying cases where
multiple cross-platform applications will be built under different
toolchain contexts.
Change-Id: I313a44fd978120ee2c60cfc7c36217e3cb377ccc
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/36602
Commit-Queue: Armando Montanez <amontanez@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
