Previously, librtld_db just hardcoded /libexec/ld-elf.so, which isn't
correct for processes that aren't using the native ABI. With this change,
librtld_db can be used to inspect non-native processes; in particular,
dtrace -c now works for 32-bit executables on amd64.
MFC after: 1 month
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
