dtrace, ctf: build sdtstubs and CTF after sdtinfo; sdtinfo follows modpost

dtrace, ctf: build sdtstubs and CTF after sdtinfo; sdtinfo follows modpost

In the following commit, sdtinfo generation will begin to
rewrite the .o file.  When this is happening, we cannot
have sdtstubs generation, dwarf2ctf, or modpost run in parallel
with it, since they all read the .o file (in modpost's case, for
symbol versions; in sdtstubs' case, for the list of symbols;
in dwarf2ctf's case, for the debugging info).  So have both of
the latter depend on the sdtinfo.c files that the sdtinfo pass
generates.  We force sdtinfo generation to run after modpost
by depending on the .mod.c files modpost generates.

For the CTF side of things, we use an order-only prerequisite
because sdtinfo's changes do not change anything which might
require dwarf2ctf to rerun, and so as not to disturb $^, which
we use to build the list of files dwarf2ctf should run over.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Acked-by: Kris Van Hees <kris.van.hees@oracle.com