Constify target_so_ops::bfd_open

Constify target_so_ops::bfd_open

This constifies the bfd_open method of struct target_so_ops.

gdb/ChangeLog
2018-02-14  Tom Tromey  <tom@tromey.com>

	* solist.h (struct target_so_ops) <bfd_open>: Make pathname
	const.
	(solib_bfd_open): Make pathname const.
	* solib.c (solib_bfd_open): Make pathname const.
	* solib-spu.c (spu_bfd_fopen): Make name const.
	(spu_bfd_open): Make pathname const.
	* solib-darwin.c (darwin_bfd_open): Make pathname const.
	* solib-aix.c (solib_aix_bfd_open): Make pathname const.