<sys/cdefs.h>: Decay expression passed to fallback version of __generic()

This ensures that __generic() more closely matches _Generic() when
using the fallback version when _Generic() is not available (such as
GCC).

Co-authored by: jrtc27
Reviewed by: jrtc27
Differential Revision: https://reviews.freebsd.org/D38215

(cherry picked from commit 9d2f9798175586bdcd3423443804fbbc0cffb8d2)