Drop special way of getting inferior context after a Cygwin signal

Drop special way of getting inferior context after a Cygwin signal

Simplify Cygwin signal handling by dropping the special way of getting
inferior context after a Cygwin signal.

I think the reason this existed was because previously we were not
able to unwind through the alternate stack used by _sigfe frames, so
without the hint of the "user" code IP, the backtrace from a signal
was unusable.

Now we can unwind through _sigfe frames, drop all this complexity.

(Restoring this specially obtained context to the inferior (as the
code currently does) skips over the actual signal delivery and
handling.  Cygwin has carried for a long time a patch which clears the
ContextFlags in the signal context, so we never attempt to restore it
to the inferior, but that interfers with gdb's ability to modify that
context e.g. if it decides it wants to turn on FLAG_TRACE_BIT.)

Change-Id: I214edd5a99fd17c1a31ad18138d4a6cc420225e3