docs(turbopack): Document build dependency on clang for rocksdb

[bgw]

Noticed this after rebuilding on top of #71688

Most Linux distributions default to gcc for their default cc implementation (e.g. via Debian's build-essential meta-package, which we can probably implicitly assume is installed), which means that they won't typically have clang installed.

librocksdb-sys seems to always depend on clang on Linux. It looks like this might be a consequence of using rust-bindgen (though I'm confused why this didn't come up earlier), as it looks like rocksdb can otherwise build with gcc.

This shouldn't be an issue for macos, since clang is the default cc on that platform.

[bgw]

• docs(turbopack): Document build dependency on clang for rocksdb #72493 👈
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