Fix to 'gdb detach' stub

Fix to 'gdb detach' stub

With this patch, 'gdb detach' correctly resumes the inferior execution
after detaching the debugger.
The bug was caused by qemu asking gdb to execute a syscall (isatty)
after the detach, and then waiting (forever) for the reply. I fixed this
by properly setting gdb_syscall_mode appropriately in the 'detach'
packet handling, so subsequent syscalls are solved by qemu rather than gdb.

Signed-off-by: Daniel Gutson <dgutson@codesourcery.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>