cdefs.h: Remove pre gcc 5.0 support

[imp]

This obsoletes https://reviews.freebsd.org/D45650 btw and also fixes the problem.

[brooks]

sys/sys/cdefs.h

235–237

Its this still relevant?

311–313

This is one case where I might consider an if,define,else,error,endif as this is used as a correctness assert and falling through could lead to a security vulnerability.

[imp]

sys/sys/cdefs.h

235–237

I noticed this and fixed it separately. I've not posted a review for it yet.

311–313

Can't do the #error here, can't embed it in a marco expansion.
However, if there's no __builtin_unreachable, we'll get an error where it's used, which is what we want. It's not needed for any .h file in the system except kassert.h where it's a macro used by C code (which has to have a supported compiler) or nvmf_tcp.h which technically has inlines, but won't be used by anybody wanting to use another compiler to build anything except the in-tree code, which needs a supported compiler.
So this gives the same 'won't compile' feature that you want at use point that you're requesting already.

[brooks]

sys/sys/cdefs.h

311–313

makes sense