stand: Go back to a.out format for /boot/loader

Turns out there's two hidden a.out dependencies. pxeldr.S assumes it has
access to the a.out header from /boot/loader and cdboot.S assumes that
/boot/loader is also a.out and doesn't use boot2.

So, go back to making a.out files for these and adjust the size checks
to use ls, but we only need to check loader.bin. Trim the size we check
against by 2,000. The difference in size between loader and loader.bin
is about 3000 bytes, but clang15 produces binaries that are a smidge
bigger so we need to relax the check just a little and accept some
additional risk for the moment.

Add some comments to loader's Makefile about this.

Sponsored by: Netflix
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D36142