MFC r265631:
Re-apply r248644. This fixes an annoying problem which caused dtrace -c to
fail to attach to stripped binaries. With the _r_debug_postinit symbol,
dtrace(1) can now set a breakpoint in the victim process after it has
registered its DOF table(s) with the kernel. r_debug_state cannot be used
for this purpose since it is called before DOF is made available, in which
case dtrace(1) cannot create USDT probes before the program begins
execution.