Set llc and opt commands on all platforms

Summary:
LLVM llc and opt commands should be set on all platforms, including
Windows. If they're not, GHC tries to execute an unnamed executable,
resulting in error messages such as:

    Error (figuring out LLVM version): : runInteractiveProcess: invalid argument (Invalid argument)
    <no location info>:
        Warning: Couldn't figure out LLVM version!
                 Make sure you have installed LLVM

This regression was introduced in e6bfc596.

Test Plan: Build GHC and test if --info shows sensible values of "LLVM llc command" and "LLVM opt command"

Reviewers: austin, #ghc

Reviewed By: austin, #ghc

Subscribers: austin

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D190

GHC Trac Issues: #7143