Don't omit bsd.compiler.mk for the nested delete-old in buildworld.

src.opts.mk will need bsd.compiler.mk to determine if CXX is
supported. Without this, src.opts.mk always marks CXX as broken and
attempts to delete all dependencies of MK_CXX from WORLDTMP.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D36890