cdefs.h: Assume the compiler supports at least GNU C 3.0 extensions

All compilers that can build FreeBSD binaries (as opposed to the entire
system) support at least gcc 9 (gcc, clang, tcc). Even pcc supports most
of the gcc 4.3 attributes. Make this file simpler by removing support
for pre-3.0 compilers.

Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D45653
Sponsored by: Netflix