Export NDK libclang path for bindgen

[StantonMatt]

Fixes #201.

This exports LIBCLANG_PATH when the selected NDK contains a host libclang shared library, so bindgen/clang-sys can find the NDK-provided libclang without extra user configuration.

The lookup covers the host layouts I found in current and older NDK prebuilts:

• lib for current Linux/macOS layouts
• lib64 for older Linux/macOS layouts
• bin for Windows libclang.dll
• musl/lib for Linux musl prebuilts, preferred first on musl hosts

Verification:

• cargo fmt --all -- --check
• cargo test cargo::tests --locked
• cargo test --locked
• cargo clippy -- -D warnings
• git diff --check

[bbqsrc]

@StantonMatt conflicts, please fix

[StantonMatt]

Rebased onto current main; the PR is mergeable again at e77bccf. The new hosted CI run is green.