Allow user not to install llvm shared object

[Keruspe]

Current rust nightly installs /usr/lib/libLLVM-7.so. This will cause conflict with the system-provided llvm as soon as llvm 7 is packaged.
It is thus impossible to use the rust-provided llvm in a future-proof way for system packages, making it mandatory to use the system-provided llvm (lacking rust patches).

It would be great to provide a way to use static linking like before (even if that means duplicating work with the thinLTO stuff).

[nagisa]

We probably should link LLVM into the backend shared library statically.

[cuviper]

This is a consequence of #53245 which started linking LLVM tools dynamically and #53828 which moved libLLVM to the main rustc component to be easily shared. I believe the codegen-backends are still linked statically to LLVM though.

[cuviper]

A simple change would be to just add a custom suffix to Rust's LLVM build, something like:

cmake -DLLVM_VERSION_SUFFIX=-rust

[Keruspe]

Something like that would be great.
Making that configurable in cargo.toml would be even better (so that nightly, beta and stable can still be installed at the same time without colliding with each other)