Another portabilty regression: before Platform we used to use elf_OBJ_FORMAT:

 #if linux_TARGET_OS || freebsd_TARGET_OS || openbsd_TARGET_OS || solaris2_TARGET_OS
 #define elf_OBJ_FORMAT 1
 #endif

which defaults to undefined on unknown platforms.  Defaulting to
non-ELF is correct, it just means that we won't rely on ELF-specific
functionality.  I've added a comment to explain that.