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
with r265456 _r_debug_postinit can be used for RD_POSTINIT events. rtld(1)
uses r_debug_state for dl state transitions, so we use its address for
RD_DLACTIVITY events.
MFC after: 2 weeks