[Headers] Replace __need_STDDEF_H_misc with specific __need_ macros

Apple needs __need_ macros for rsize_t and offsetof. Add those, and also ones for nullptr_t, unreachable, max_align_t.

Reviewed By: aaron.ballman, efriedma, #libc, ldionne

Differential Revision: https://reviews.llvm.org/D157757