stand/kboot: Fix the linker script OUTPUT_FORMAT

ld.bfd doesn't understand elf64-aarch64 but does have
elf64-littleaarch64. Switch to this so we can link kboot with it.

While here switch to the single format version. We are unlikely to
support booting from a big-endian Linux.

Reviewed by: imp, emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45258

(cherry picked from commit 8e82c5e28db6fb19f75d10fd2e7c3db784723f1f)