rustbuild: Enable cross-compiling LLVM

rustbuild: Enable cross-compiling LLVM

Currently all multi-host builds assume the the build platform can run the
`llvm-config` binary generated for each host platform we're creating a compiler
for. Unfortunately this assumption isn't always true when cross compiling, so we
need to handle this case.

This commit alters the build script of `rustc_llvm` to understand when it's
running an `llvm-config` which is different than the platform we're targeting for.