Implement a variant of the existing gdb_do_syscall() which
takes a va_list.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Christopher Covington <cov@codeaurora.org>
Message-id: 1439483745-28752-4-git-send-email-peter.maydell@linaro.org