cdefs.h: remove intel_compiler support

The age of the intel compiler support is so old as to be
uninteresting. No recent recports of intel compiler support have been
received. Remove all the special case workarounds for the Intel
compiler. Should there be interest in supporting the compiler, contact
me and I'll work with people to make it happen, though I suspect these
instances are more likely to be in the way than to be helpful.

Reviewed by: cem, emaste, vangyzen, dim
Differential Revision: https://reviews.freebsd.org/D26817