[Bazel] Rework LLVM target selection

This patch introduces a custom rule for expanding the LLVM target
enumeration .def files. This provides a slightly cleaner API for these
rules, but is mostly to permit selects to be used when determining which
LLVM targets to build. Right now the target list is generated at Bazel
configure time, but this will allows us to add functionality to also
control which targets are built based on config settings.

Tested: Ran `bazel test --config=rbe ... @llvm-project//...`

Reviewed By: chandlerc

Differential Revision: https://reviews.llvm.org/D104969