Move 'enum bfd_endian' to a non-generated header

Because:

- GDB uses enum bfd_endian extensively.
- gdbserver does not build/link-with bfd.
- We'd like to share more code between gdb and gdbserver.

It'd make our lives easier if we could just use bfd_endian in
gdbserver as well.

The problem is that bfd_endian is defined in a header that only exists
if bfd is built/configured.

Thus this moves bfd_endian to a separate header, so gdbserver can
include it.

bfd/ChangeLog:
2016-02-11  Pedro Alves  <palves@redhat.com>

	* bfd-in.h: Include bfd-types.h
	* bfd-in2.h: Regenerate.
	* targets.c (enum bfd_endian): Moved to include/bfd-types.h.

include/ChangeLog:
2016-02-11  Pedro Alves  <palves@redhat.com>

	* bfd-types.h: New file.