libiberty: fix lrealpath on Windows NTFS symlinks

gcc computes the wrong prefix if invoked through an NTFS
symlink. Try to resolve it if possible.

PR/108350

libiberty/ChangeLog:

	* lrealpath.c (lrealpath): try to resolve symlink and
	use UNC paths where applicable.

Signed-off-by: Jonathan Yong <10walls@gmail.com>