gdb: const-ify hostio methods parameter in remote.c

gdb: const-ify hostio methods parameter in remote.c

gdb/ChangeLog:

	* remote.c (class remote_target) <remote_hostio_send_command,
	remote_hostio_parse_result>: Constify parameter.
	(remote_hostio_parse_result): Likewise.
	(remote_target::remote_hostio_send_command): Adjust.
	(remote_target::remote_hostio_pread_vFile): Adjust.
	(remote_target::fileio_readlink): Adjust.
	(remote_target::fileio_fstat): Adjust.

Change-Id: I6b585b99937e6526a0a7e06261d2193114589912