Guard offsetof

[cy]

Summary

While implementing a gets_s(3) libc function, which requires rsize_t from stddef.h, offsetof from sys/stddef.h conflicts with the one in stddef.h. This diff proposes to add a guard around offsetof similar to the ptrdiff_t guard implemented by r221476 and r270277

[ed]

What about making the macros identical? That way there is no need for a guard. s/field/member/

[cy]

That makes more sense. Thank you. I'll commit that later today or tomorrow.