Revert r316779:
Remove (yet again) the definition for the GCC __nonnull() attribute.

While GCC's nonnull attribute is generally useful to prevent misuse of
some functions it also tends to do rather dangerous "optimizations". Now
that we have replaced all such uses with the clang nullability qualifiers,
the GCC attribute is unnecessary.

The version was left around and kept for 11.1-Release for old
versions of GCC from ports. Newer versions of GCC are not attempting to
"fix" the system includes so this should not have any side effect and
matches what we do in -current.