gdbserver: Don't send a GDB syscall until the system CPU is stopped

gdbserver: Don't send a GDB syscall until the system CPU is stopped

Fix an issue where the GDB server implementation was sending GDB syscall
requests while the system CPU was still running.  Syscall requests must
be sent while the CPU is stopped otherwise replies from the GDB client
might get dropped and the GDB server might be incorrectly transitioned
into a 'RUN_STATE_PAUSED' state.

Signed-off-by: Meador Inge <meadori@codesourcery.com>
[PMM: trivial rebase, reinstated comma after last item in RSState enum]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>