Try to use the canonical version of a sysroot for debug file links.

Try to use the canonical version of a sysroot for debug file links.

Object file paths passed to find_separate_debug_file are always
canonical paths with symbolic links resolved.  If a sysroot path
traverses a symbolic link, it will not match the object file paths.
Generate a canonical version of the sysroot directory.  If it is
valid, use it instead of gdb_sysroot with child_path to determine if
an object file is under a system root.

gdb/ChangeLog:

	* symfile.c (find_separate_debug_file): Use canonical path of
	sysroot with child_path instead of gdb_sysroot if it is valid.