dtrace: support SDT in single-file modules

dtrace: support SDT in single-file modules

A limitation of long standing in the SDT machinery is that it didn't
support modules that were implemented entirely in one file.  This is
because the SDT stubs generation machinery (which emits zero-byte
assembler stubs for all DTrace probe points) was implemented at the
point at which multi-file modules are linked into a single file prior to
modpost.  Since single-file modules *have* no such stage, they never got
their stubs, and probes never fired.  This becomes essential to fix so
that perf_events probes will work, because some of those are in
single-file modules.

We fix it by tearing out the sdtstub code from its prior home in
Makefile.build, and moving it into the late modpost stage, at the same
point at which sdtinfo files are generated.  We use the same prerequites
trick as is used for CTF files to link the sdtstub files into the
resulting modules; because this all happens at the very last possible
moment, it shares the same code path for single- and multifile modules,
and is a good bit simpler to boot (no need to manipulate the link
process or transform filenames intricately during stub generation).

There is one additional wrinkle: because the dtrace probes are now
always undefined until the last minute, we must adjust modpost to
disregard all undefined symbols that start "__dtrace_probe_".

Orabug: 23316392
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Acked-by: Kris Van Hees <kris.van.hees@oracle.com>