gdb: Remove support for SH-5/SH64

gdb: Remove support for SH-5/SH64

Since bfd dropped support for SH-5, there's no point in keeping it in
GDB either.

This restores --enable-targets=all builds.

gdb/ChangeLog:
2018-04-16  Pedro Alves  <palves@redhat.com>

	* MAINTAINERS (sh): Remove.
	* Makefile.in (ALL_TARGET_OBS): Remove sh64-tdep.o.
	(HFILES_NO_SRCDIR): Remove sh64-tdep.h.
	(ALLDEPFILES): Remove sh64-tdep.c.
	* NEWS: Mentions that support for SH-5/SH64 is removed.
	* configure.tgt (sh*-*-linux*): Remove reference to sh64-tdep.o.
	(sh*-*-openbsd*): Ditto.
	(sh64-*-elf*): Remove.
	(sh*): Remove.
	* regcache.c (cooked_write_test): Remove bfd_mach_sh5 case.
	* sh-linux-tdep.c: Remove reference to bfd_mach_sh5.
	* sh-tdep.c: No longer include "sh64-tdep.h".
	(sh_gdbarch_init): Remove reference to bfd_mach_sh5.
	* sh64-tdep.c, sh64-tdep.h: Remove files.