rebuild passes_list.md does not function in OSS

**Describe the bug**

Post https://github.com/google/xls/pull/2531, The new passes_list.md file is meant to be automatically regenerated using the //xls/passes/tools:rebuild_documentation target. Unfortunately due to differences in how the build works between google and oss this does not work in the oss version. This is due to inside google the libcxx being a normal dependency resolved by bazel but externally it uses the compiler to provide this library. This causes issues since the libtooling binary that rebuild_documentation uses internally does not know how to find the library (and the libcxx is tightly bound to the exact clang version so it might not work even if it did find it).

If libcxx were buildable in bazel this would probably be fixable by manually adding it to the search paths like we do for some of the other libc headers. Unfortunately there is no OSS bazel build for libcxx.

It's likely this could be hacked together by building libcxx separately and pointing rebuild_benchmarks to its files.

For now however the rebuild command will act like the bazel rule doc where the generation is done internally at google until somebody has the time to fix this up.