clang_configure: add /usr/lib/libclang.so to search paths

[trnila]

On ArchlinuxARM, running clang --version returns:

clang version 20.1.8
Target: aarch64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

compared to Ubuntu:

Ubuntu clang version 14.0.0-1ubuntu1.1
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

which breaks the build with:

@@com_github_mvukov_rules_ros2+//repositories/rust:bindgen_toolchain_impl depends on @@com_github_mvukov_rules_ros2++non_module_deps+rules_ros2_config_clang//:libclang in repository @@com_github_mvukov_rules_ros2++non_module_deps+rules_ros2_config_clang which failed to fetch. no such package '@@com_github_mvukov_rules_ros2++non_module_deps+rules_ros2_config_clang//': Failed to fetch libclang_path:
realpath: '/usr/lib/llvm-aarch64-unknown-linux-gnu'$'\n''Thread/lib/libclang.so.1': No such file or directory

On ArchlinuxARM, libclang.so is installed directly under /usr/lib, so theres no need to get the version first. This PR adds a search for libclang.so in /usr/lib first, and falls back to the existing lookup if it’s not found.

I also evaluated a more hermetic approach using toolchains_llvm.
Theres @llvm_toolchain_llvm//:bin/clang and @llvm_toolchain_llvm//:lib/libclang.so target, but it can be added to the root bazel module only. Another issue is size - extracted toolchain is about 6 GB and takes more than 400 seconds to extract on Raspberry Pi - not sure if its worth it. Maybe there could be some rules_ros2 option to provide rules_ros2_config_clang replacement if root module is already using toolchains_llvm...

[mvukov]

Hi, IIRC there was a specific reason why I implemented it this way. It could be that it was needed for arm64 platform -- e.g. on nvidia jetson. We'll need to double-check this.