Make cddl/osnet/lib/libdtrace buildable with MKLLVM=yes

Clang/LLVM does not recognize:
- -Wno-maybe-uninitialized
- -Wno-unused-but-set-variable

MKGCC=yes still builds the distribution.

This is a step forward functional MKLLVM=yes HAVE_LLVM=yes build.

Sponsored by <The NetBSD Foundation>

(kamil)