Do not manipulate "target:" filenames as local paths

Do not manipulate "target:" filenames as local paths

This commit alters two places that manipulate object file filenames
to detect "target:" filenames and to not attempt to manipulate them
as paths on the local filesystem:

 - allocate_objfile is updated to not attempt to expand "target:"
   filenames with gdb_abspath.

 - load_auto_scripts_for_objfile is updated to not attempt to load
   auto-load scripts for object files with "target:" filenames.

gdb/ChangeLog:

	* objfiles.c (allocate_objfile): Do not attempt to expand name
	if name is a "target:" filename.
	* auto-load.c (load_auto_scripts_for_objfile): Do not attempt
	to load auto-load scripts for objfiles with "target:" filenames.