dtrace: is-enabled probes for SDT

dtrace: is-enabled probes for SDT

This is the module side of the is-enabled probe implementation.  SDT
distinguishes is-enabled probes from normal probes by the leading ? in
their sdpd_name; at probe-firing time, the arch-dependent code arranges
to return 1 appropriately.

On x86, also arrange to jump past the probe's NOP region.  There was no
need to do this before now, because a trap followed by a bunch of NOPs
is a perfectly valid instruction stream: but is-enabled probes have a
three-byte sequence implementing "xor %rax, %rax", and overwriting only
the first byte of that leaves us with a couple of bytes that must be
skipped.  On SPARC, we drop the necessary return-value-changing
instruction into the delay slot of the call that used to be there
before we overwrote it with NOPs;: the instruction already there
is setting up the function argument-and-return-value, which is 0
when the probe is disabled, so we can overwrite it safely.

(We make minor adjustments to allow sdt_provide_probe_arch() to
safely modify the sdp_patchpoint.)

Finally, add a test use of an is-enabled probe to dt_test, used by the
DTrace testsuite.

[nca: sparc implementation, ip address adjustment, commit msg]
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Acked-by: Kris Van Hees <kris.van.hees@oracle.com>
Orabug: 25143173