cdefs: add bounds-safety annotation macros

[abhijeetsharma2002_gmail.com]

Summary

Define the -fbounds-safety vocabulary (counted_by, sized_by,
__single, the forge escape hatches). Every macro expands to nothing
under compilers without the extension, so the annotated tree builds
unchanged with the stock toolchain.

Sponsored by: The FreeBSD Foundation

[imp]

Ah! I complained that these were missing from a prior review, but they are just what I was hoping for.