cdefs cleanup

[Warner Losh]

I've put up the following three reviews
https://reviews.freebsd.org/D26817
https://reviews.freebsd.org/D26818
https://reviews.freebsd.org/D26819

which cleanup sys/cdefs.h in three ways:
'17 removes support for the ancient Intel Compiler.
'18 requires the compiler's preprocessor support the __GNUC__ extensions
'19 removes support for gcc prior to 4.2

Please comment on the reviews, or any general issues here. At this point, I
think nobody would notice if I just committed them, but this file is rather
central to things.... so I'm asking around a bit first.

Warner