Add used to CTOR_LIST__ and DTOR_LIST

Enabling BSD_CRTBEGIN on amd64 resulted in
error: unused variable 'CTOR_LIST'.

CTOR_LIST is indeed unused in crtbegin.c; it marks the beginning of
the .ctors array and is used in crtend.c. Annotate DTOR_LIST as
well for consistency.

Discussed with: andrew
MFC with: r339738
Sponsored by: The FreeBSD Foundation