Fix a gdbremote bug in _M/_m stub support detection.

When a stub reported $#00 (unsupported) for _M and _m
packets, the unsupported response was not handled and
the client then marked the _M/_m commands as definitely
supported.  However, they would always fail, preventing
lldb's fallback InferiorCallMmap-based allocation strategy
from being used to attempt to allocate memory in the inferior
process space.