obsolete xc16x

The xc16x md_apply_fix code is just so broken that in my opinion the
target should never have been accepted, and from a quick look at
commit logs for the target it appears that no one has ever contributed
fixes for anything.  This target has just been a 14 year burden on
global binutils and cgen maintainers.  That's not how free software is
supposed to work.

bfd/
	* config.bfd: Obsolete xc16x.
gas/
	* config/tc-xc16x.c (md_apply_fix): Add FIXME.