stapsdt: initial support for probing sdt notes.

For parsing ktap scripts, uprobes functions are used for stapsdt probes
too, the only differ between then is type argument, which is bypassed up
to find_symbol() that's it.

Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>
scripts: add test script for stapsdt probes

The only library in /lib/x86_64-linux-gnu that have sdt notes is
libgcc_s on by machine, with unwind marker.
So this patch add a simple ktap script that use it.

Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>
sdt: replace "stapsdt" to just "sdt"

It more shorter, and simpler.

Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>
stap: fix possible segmentation fault without symbol

As Jovi noticed:
    Segmentation fault for "trace sdt:/lib64/libc.so.6 {...}"
    No symbol given in that case.

Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>