removing USDT's libelf dependency

[Mark Johnston]

Hello,

At the moment, any FreeBSD program containing USDT probes needs to be
linked with libelf. This is so that drti.o can find the address of the
DOF section within the executable or library. With this, it can extract
the DOF and load it into the kernel prior to beginning execution.

This requirement is an annoyance since it's specific to FreeBSD and
many upstream projects which support DTrace aren't aware of it, so some
patching is necessary to get their probes working on FreeBSD. It also
increases the startup cost of programs containing USDT probes, which can
be significant for e.g. sh(1) or libc/libthr (plockstat).

Solaris exploits a feature of its linker to solve this problem - the
dtrace -G step emits an object file containing a symbol called
__SUNW_dof; the linker knows to fill in the value of this symbol with
the address of the DOF section, so drti.o begins execution with the DOF
section already available.

It turns out that GNU ld implements a similar feature, albeit in a more
restrictive manner: a symbol named __start_<id> will have its value
filled in with the address of the section named <id>. The catch is that
<id> has to be a valid C identifier, and the DOF section is
conventionally named ".SUNW_dof", which of course doesn't quite work. I
propose using "SUNW_dof" instead so that we can make use of this linker
feature and eliminate the libelf dependency.

DOF sections have their own ELF section type (SHT_SUNW_dof), so
anything that might want to find a DOF section should already be
searching for that rather than the conventional name. Depending on this
linker feature is also arguably a portability concern given that lld is
on the horizon, but we depend on it already anyway - this feature is
used in FreeBSD's linker set implementation (which in turn is used by
our SDT code :). So I don't think this would introduce a new problem.

Any thoughts?

Thanks,
-Mark