gdbstub: Don't use GDB syscalls if no GDB is attached

In two places in gdbstub.c we look at gdbserver_state.init to decide
whether we're going to do a semihosting syscall via the gdb remote
protocol:
 * when setting up, if the user didn't explicitly select either
   native semihosting or gdb semihosting, we autoselect, with the
   intended behaviour "use gdb if gdb is connected"
 * when the semihosting layer attempts to do a syscall via gdb, we
   silently ignore it if the gdbstub wasn't actually set up

However, if the user's commandline sets up the gdbstub but tells QEMU
to start rather than waiting for a GDB to connect (eg using '-s' but
not '-S'), then we will have gdbserver_state.init true but no actual
connection; an attempt to use gdb syscalls will then crash because we
try to use gdbserver_state.c_cpu when it hasn't been set up:

#0  0x00007ffff6803ba8 in qemu_cpu_kick (cpu=0x0) at ../../softmmu/cpus.c:457
#1  0x00007ffff6c03913 in gdb_do_syscallv (cb=0x7ffff6c19944 <common_semi_cb>,
    fmt=0x7ffff7573b7e "", va=0x7ffff56294c0) at ../../gdbstub.c:2946
#2  0x00007ffff6c19c3a in common_semi_gdb_syscall (cs=0x7ffff83fe060,
    cb=0x7ffff6c19944 <common_semi_cb>, fmt=0x7ffff7573b75 "isatty,%x")
    at ../../semihosting/arm-compat-semi.c:494
#3  0x00007ffff6c1a064 in gdb_isattyfn (cs=0x7ffff83fe060, gf=0x7ffff86a3690)
    at ../../semihosting/arm-compat-semi.c:636
#4  0x00007ffff6c1b20f in do_common_semihosting (cs=0x7ffff83fe060)
    at ../../semihosting/arm-compat-semi.c:967
#5  0x00007ffff693a037 in handle_semihosting (cs=0x7ffff83fe060)
    at ../../target/arm/helper.c:10316

You can probably also get into this state via some odd
corner cases involving connecting a GDB and then telling it
to detach from all the vCPUs.

Abstract out the test into a new gdb_attached() function
which returns true only if there's actually a GDB connected
to the debug stub and attached to at least one vCPU.

Reported-by: Liviu Ionescu <ilg@livius.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Message-id: 20220526190053.521505-2-peter.maydell@linaro.org