sys/cdefs.h: Note gcc supports many __has_* macros

The has_* macros are no longer clang-speicifc. gcc 5 introduced
has_include, gcc 9 introduce has_attribute, gcc 10 introduced
has_builtin. So all of these are supported by all the versrions of gcc
we support as a project (we have gcc 10 and newer ports, but no older
ones), so just refer to them as supported by gcc. The exact version
doesn't matter so much these days (we provide a fallback for older
compilers still, should anybody still be using them), so don't mention
them in the comments (but here in the commit message they are
discoverable).

gcc 14 adds has_feature and has_extension, so note that since it
will be relevant for many years.

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