build: process the libxml2 library path for embedding

Process the path for libxml2 before embedding that into the command line
that is generated in `llvm-config`.  Each element in the path is being
given a `-l` unconditionally which should not be the case for absolute
paths.  Since the library path may be absolute or not, just apply some
CMake pre-processing when generating the path.

Before:
```
/usr/lib/x86_64-linux-gnu/libz.so -lrt -ldl -ltinfo -lpthread -lm /usr/lib/x86_64-linux-gnu/libxml2.so
```

After:
```
/usr/lib/x86_64-linux-gnu/libz.so -lrt -ldl -ltinfo -lpthread -lm -lxml2
```

Resolves PR44179!