Move EM_XTENSA to uapi/linux/elf-em.h

Move EM_XTENSA to uapi/linux/elf-em.h

This should never have been defined in the arch tree to begin with,
and now uapi/linux/audit.h header is going to use EM_XTENSA
in order to define AUDIT_ARCH_XTENSA which is needed to implement
syscall_get_arch() which in turn is required to extend
the generic ptrace API with PTRACE_GET_SYSCALL_INFO request.

Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Elvira Khabirova <lineprinter@altlinux.org>
Cc: Eugene Syromyatnikov <esyr@redhat.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: linux-xtensa@linux-xtensa.org
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>