Handle unwinding from SEGV on Windows

Handle unwinding from SEGV on Windows

PR win32/30255 points out that a call to a NULL function pointer will
leave gdb unable to "bt" on Windows.

I tracked this down to the amd64 windows unwinder.  If we treat this
scenario as if it were a leaf function, unwinding works fine.

I'm not completely sure this patch is the best way.  I considered
having it check for 'pc==0' -- but then I figured this could affect
any inaccessible PC, not just the special 0 value.

No test case because I can't run dejagnu tests on Windows.  I tested
this by hand using the test case in the bug.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30255