librtld_db: Use the auxv to figure out where to look up loader symbols.

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