Allow configuring codegen-backends directory name

[Keruspe]

Currently, codegen backends are installed as e.g. /usr/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_trans-llvm.so

As there is no hash or such in the library name (like there is for all the rust libraries shipped with rustc), installing several versions of rust is not possible as there would be a file conflict.
On exherbo we can install rust stable, rust beta and rust nightly, but with these codegen backends we cannot anymore. It would be great to either have a hash in the library name, or to be able to change codegen-backends to e.g. codegen-backends-beta in config.toml at build time, for example.

Would such a patch be accepted? Which of the two options would be preferred?