META MODE: Rework circular dependency guard for librtld_db/libproc.

librtld_db only needs libutil.h to build, not the libproc library. So
it can safely use its header and allow libproc to depend on librtld_rb
to be built first to link. This is required after fixing ld --sysroot
in r291226.

Sponsored by: EMC / Isilon Storage Division