'spack compiler add' resolves relative path to absolute path (#11792)

Fixes #11782

Spack was not properly resolving relative paths to absolute paths
when a relative path was passed to "spack compiler add [PATH]".
Now, if provided a relative path, the absolute path is written to
compilers.yaml rather than the relative path.

lib/spack/llnl/util/filesystem.py

@@ -1443,6 +1443,7 @@ def search_paths_for_executables(*path_hints):
         if not os.path.isdir(path):
             continue
 
+        path = os.path.abspath(path)
         executable_paths.append(path)
 
         bin_dir = os.path.join(path, 'bin')