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.

MFC after:	2 weeks