Fix llvm-config --system-libs output on FreeBSD and NetBSD

Summary:
For various reasons, CMake's detection mechanism for `backtrace()`
returns an absolute path `/usr/lib/libexecinfo.so` on FreeBSD and
NetBSD.

Since `tools/llvm-config/CMakeLists.txt` only checks if system
libraries start with `-`, this causes `llvm-config --system-libs` to
produce the following incorrect output:

```
-lrt -l/usr/lib/libexecinfo.so -ltinfo -lpthread -lz -lm
```

Fix it by removing the path and the `lib` prefix, to make it look like a
regular short library name, suitable for appending to a `-l` link flag.

This also fixes the `Bindings/Go/go.test` test case, since that always
died with "unable to find library -l/usr/lib/libexecinfo.so".

Reviewers: chandlerc, emaste, joerg, krytarowski

Reviewed By: krytarowski

Subscribers: hans, bdrewery, mgorny, hintonda, llvm-commits

Differential Revision: https://reviews.llvm.org/D42702

llvm-svn: 326358