[llvm-config] Fix bug where `--libfiles` and `--names` would produce

incorrect output when LLVM is built with `LLVM_BUILD_LLVM_DYLIB`.

`llvm-config` previously produced output like this

```
$ llvm-config --libfiles
/usr/lib/liblibLLVM-4.0svn.so.so
$ llvm-config --libnames
liblibLLVM-4.0svn.so.so
```

The library prefix and shared library extension were added to
the library name twice which was wrong.

I wanted to write a test cases for this but it looks like **all**
`llvm-config` tests were disabled by r260386 so I'll leave this for
now.

Subscribers: llvm-commits, tstellarAMD

Reviewers: beanz, DiamondLovesYou, axw

Differential Revision: https://reviews.llvm.org/D27393

llvm-svn: 289488