Add experimental support for LLVM 3.6

[digitalextremist]

Succeeded in builds in my experience.

@brixen would know better than I would the actual consequences of this.

[yorickpeterse]

A big pile of changes for LLVM 3.6 can already be found in this pull request: https://github.com/rubinius/rubinius/pull/3367/files

Sadly just changing the version to build with isn't enough as LLVM 3.6 removed the old JIT API that we rely on. I'll close this issue in favour of #3367.

[digitalextremist]

@yorickpeterse, damn. I thought this was solvable by magic... but alas, it is not. I reverted to 14.04 of Ubuntu while this is sorted out.

[yorickpeterse]

@digitalextremist If available you can install a 3.5 version of LLVM (usually distributions keep a few versions around) and use ./configure --llvm-config=llvm-config3.5 (or something similar) to use an older version of LLVM.

[digitalextremist]

It'd be great if the configure script could detect older versions. I tried to rvm install rbx on a fresh Ubuntu 15.04 x64 install, and it just choked. That's probably a big problem as people migrate to the next distribution of that Linux distribution.

[jc00ke]

Yeah, some detection around llvm-config (maybe it would fall back to llvm-config-3.5 then to llvm-config-3.4) would be nice.

[larskanis]

Install with RVM worked for me on Ubuntu-15.04:

sudo apt-get install llvm-3.5
rvm install rbx-2.5.8 -C --llvm-config=llvm-config-3.5