gdb: change xml_fetch_another a function_view

The xml_fetch_another is currently a plain function pointer type, with a
`void *` baton parameter.  To improve type-safety, change this to a
function_view.  Any required data is captured by a lambda at the call
site.

gdb/ChangeLog:

	* xml-support.h (xml_fetch_another): Change type to be a
	function_view.
	(xml_process_xincludes): Remove baton parameter.
	(xml_fetch_content_from_file): Change baton parameter to
	dirname.
	* xml-support.c (struct xinclude_parsing_data)
	<xinclude_parsing_data>: Remove baton parameter.
	<fetcher_baton>: Remove.
	(xinclude_start_include): Adjust.
	(xml_process_xincludes): Adjust.
	(xml_fetch_content_from_file): Replace baton parameter with
	dirname.
	* xml-syscall.c (syscall_parse_xml): Remove baton parameter.
	(xml_init_syscalls_info): Use a lambda.
	* xml-tdesc.c (tdesc_parse_xml): Remove baton parameter.
	(file_read_description_xml): Use a lambda.
	(fetch_available_features_from_target): Change baton parameter
	to target_ops.
	(target_read_description_xml): Use a lambda.
	(target_fetch_description_xml): Use a lambda.
	(string_read_description_xml): Update.

Change-Id: I7ba4b8f5e97fc6a952c6c20ccc3be92a06cc2bd2