stand: Improve some debugging experience

stand: Improve some debugging experience

Some of these files using <FOO>_DEBUG defined a DEBUG() macro to serve as a
debug-printf. -DDEBUG is useful to enable some debugging output across
multiple ELF/common parts, so switch the DEBUG-as-printf macros over to
something more like DPRINTF that is more commonly used for this kind of
thing and less likely to conflict.

userboot/elf64_freebsd debugging also assumed %llx for uint64; use PRIx64
instead.

MFC after:	1 week