Disabling hardware single step in gdbserver

This patch gives gdbserver the ability to omit the 's' reply to
'vCont?'.  This tells gdb that hardware single-step is definitely not
supported, causing it to fall back to using software single-step.
This is useful for testing the earlier change to
maybe_software_singlestep.

Approved-By: Andrew Burgess <aburgess@redhat.com>