• ifndef illumos -> ifdef FreeBSD

This revision is a substantial update of the previous one.

It turns out that we don't need the linker to emit special symbols for us:
dt_link.c already goes through the trouble of putting the SUNW_dof symbol at
the beginning of the .SUNW_dof section, so we can find the address of the DOF
section simply by taking the address of SUNW_dof. No linker magic needed. In
fact, I'm now totally unclear as to why drti.o needed to be modified to use
libelf in the first place. For some reason I was convinced that we were missing
a feature of the Sun linker needed to make this trick work.

So this revision mostly just restores upstream code in drti.o without having
to change the DOF section name. The major complication is the case where
multiple dtrace-generated object files are linked into the same object. Since
the SUNW_dof symbol is global (it has to be, since it's referenced in drti.o),
linking multiple object files causes a problem, since they all contain
definitions of SUNW_dof.

I fixed this by running objcopy --localize-hidden on the generated object file,
and by giving the SUNW_dof symbol hidden visibility (which it should have
anyway). So now each generated object file has its own local SUNW_dof symbol
for its corresponding DOF section, so we no longer need to loop over the DOF
sections in drti.o (we added that because Node.js uses multiple object files).
Most of the new code is needed to run objcopy in dtrace -G, which is very
straightforward.

Testing: all usdt tests pass, ruby and postgres successfully create probes.