Move LIBADD lines from usr.bin/clang/*/Makefile one level up

Some utilities under usr.bin/clang were only linked to libz, while most
others were linked to libz and libzstd. Make this consistent, and remove
repetition, by moving these LIBADD lines to usr.bin/clang/clang.prog.mk
and usr.bin/clang/clang.prog.mk.

MFC after: 3 days