cdefs: Rework and augment visibility attributes

cdefs: Rework and augment visibility attributes

One macro based on GCC visibility functions was already defined,
__exported.  However, it was marked as available for all GCC versions
when it fact it was introduced with gcc 4.0.

There was no macro for hidden visibility attributes, so the macros
__dso_hidden and __dso_public were borrowed from NetBSD.  __dso_public
is an alias for __exported.  The definition for all three macros were
made on the condition that the gcc version used is 4.0 or later.