Rollup merge of #47846 - roblabla:bugfix-ocaml, r=kennytm

Work around LLVM OCAML binding installation failure

Hello,

I have OCaml installed on my machine, and compiling rust systematically fails when LLVM attempts installing the OCaml bindings in `/usr/lib/ocaml`, which is write-protected. Here are the logs: https://gist.github.com/roblabla/3f147914c5df627c9d97ab311ba133ad

Some digging around the issue reveals:

- The code that finds if OCaml is installed, and sets the bindings to be compiled/installed: https://github.com/llvm-mirror/llvm/blob/b24a45d2e9f4fc10c3f9e16172104910b38637f2/cmake/config-ix.cmake#L612
- https://github.com/llvm-mirror/llvm/blob/b24a45d2e9f4fc10c3f9e16172104910b38637f2/bindings/ocaml/llvm/CMakeLists.txt Some code that does the installation.

The problem seems to be that `LLVM_OCAML_INSTALL_PATH` is set to `OCAML_STDLIB_PATH` by default, which is in `/usr/lib/ocaml`, instead of the prefix.

This PR "fixes" the issue by setting `LLVM_OCAML_INSTALL_PATH` to `usr/lib/ocaml`. I haven't found a way to make LLVM not build OCaml, which would probably be a superior fix.

diff --cc src/bootstrap/native.rs
Simple merge