Temporarily change the default for use-g-packet-for-reading to false,

Temporarily change the default for use-g-packet-for-reading to false,
until we can automatically fall back to p/P if g/G are not supported;
it looks like there is a bug in debugserver's g/G packets taht needs
to be fixed, or debugserver should stop supporting g/G until that bug
is fixed.  But we need lldb to be able to fall back to p/P correctly
for that to be a viable workaround.