Allow building llvm from git; support llvm3.4svn

This adds makerules for building LLVM from latest upstream git master to be able to try out new versions.
This also adds support for the changed APIs as of yesterday's LLVM master.
Align unsafe_store!/unsafe_load semantics
Merge pull request #3983 from JuliaLang/kf/unsafestore

Align unsafe_store! semantics with unsafe_load semantics
Merge pull request #3984 from JuliaLang/kf/llvm34

Allow building llvm from git; support llvm3.4svn
Merge branch 'master' of github.com:JuliaLang/julia into jb/newvars