configure: Accept LLVM 3.5 for building rust

This is the current head of LLVM, and we can indeed build with 3.5
mk: Fix --llvm-root finding tools

LLVM's tools are not contained in the local directory if --llvm-root is used by
the ./configure script. This fixes the installation path to be the root provided
by --llvm-root.
auto merge of #12433 : alexcrichton/rust/fix-some-config-things, r=brson

These are mostly centered around using an external LLVM (notably 3.5)