gdbstub: fix gdb_get_cpu(s, pid, tid) when pid and/or tid are 0

a TID or PID value means "any thread" (resp. "any process"). This commit
fixes the different combinations when at least one value is 0.

When both are 0, the function now returns the first attached CPU,
instead of the CPU with TID 1, which is not necessarily attached or even
existent.

When PID is specified but TID is 0, the function returns the first CPU
in the process, or NULL if the process does not exist or is not
attached.

In other cases, it returns the corresponding CPU, while ignoring the PID
check when PID is 0.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20190119140000.11767-1-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>