libctf: we do in fact support foreign-endian old versions

The worry that caused this to not be supported was because we don't
bother endian-flipping version-related fields before checking them.
But they're all unsigned chars anyway, and don't need any flipping at
all.

This should be supported and should already work.  Enable it.

libctf/
	* ctf-open.c (ctf_bufopen): Don't prohibit foreign-endian
        upgrades.