Jakub Jelinek has recently introduced support in GCC for unwinding
frames with the `Sī augmentation, used to denote signal stack frames.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26208

Although GDB already does a lot of grunt work in recognizing signal
stack frames to avoid the same kind of unwinding problem, it could
take advantage of this to (a) remove machine-specific code that does
so, when the augmentation is widely available, and (b) speed up
recognizing such frames when it is only partially available.

This patch introduces code to recognize and take advantage of the "S"
augmentation, but makes no effort to remove existing machine-specific
code.  The testcase probably already worked without the change, but
it's good to have it there to avoid regressions when someone decides
to experiment with taking code out.

Ok to install?  Tested on amd64-linux-gnu.