gcc/file-prefix-map: Allow remapping of relative paths

gcc/file-prefix-map: Allow remapping of relative paths

Relative paths currently aren't remapped by -ffile-prefix-map and friends.
When cross compiling with separate 'source' and 'build' directories, the same
relative paths between directories may not be available on target as compared
to build time.

In order to be able to remap these relative build paths to paths that would
work on target, resolve paths within the file-prefix-map function using
realpath().

This does cause a change of behaviour if users were previously relying upon
symlinks or absolute paths not being resolved.

Use basename to ensure plain filenames don't have paths added.

gcc/ChangeLog:

	* file-prefix-map.cc (remap_filename): Allow remapping of relative paths.